Sort AMI names in Artifact.String - fixes random test failures
Sort AMI names in Artifact.String - fixes random test failures

With go tip, the output from Artifact.String will sometimes be output in a
different order than the tests. Sort the AMI strings before outputting.
See https://travis-ci.org/mitchellh/packer/jobs/28748467 for an example of this
failure.

rodm/packer,Banno/packer,jamesleech/packer,aneeshusa/packer,mohae/packer,pvandervelde/packer,rickard-von-essen/packer,grange74/packer,Cbeck527/packer,bryson/packer,AlphaStaxLLC/packer,stagrlee/packer,aneeshusa/packer,blairham/packer,LinusU/packer,esemplare/packer,kpettijohn/packer,mefellows/packer,marklap/packer,samdunne/packer,rodm/packer,dantran/packer,jonathankarsh/packer,icnocop/packer,dave2/packer,jszwedko/packer,youhong316/packer,hashicorp/packer,henrysher/packer,lantrix/packer,mayflower/packer,atlassian/packer,icnocop/packer,ephemeralsnow/packer,marklap/packer,legal90/packer,paulmey/packer,grubernaut/packer,arizvisa/packer,boumenot/packer,Xiol/packer,sean-/packer,evertrue/packer,c22/packer,jean/packer,grange74/packer,visioncritical/packer,dantran/packer,jonathankarsh/packer,mvanholsteijn/packer,sneal/packer,umang1912/packer,rrva/packer,tas50/packer,mefellows/packer,atlassian/packer,tb3088/packer,ricardclau/packer,monsterzz/packer,yonglehou/packer,pieter-lazzaro/packer,timsutton/packer,ephemeralsnow/packer,justincampbell/packer,monkeylittleinc/packer,jen20/packer,btd/packer,hazelesque/packer,Cbeck527/packer,atlassian/packer,jamesbishopp/packer,DualSpark/packer,higebu/packer,orivej/packer,routelastresort/packer,sgirones/packer,tracyde/packer,cyberhouse/packer,bryson/packer,trumant/packer,ChrisLundquist/packer,danschaffer/packer,bensojona/packer,mafrosis/packer,jamesbishopp/packer,mefellows/packer,dragon788/packer,quantang/packer,visioncritical/packer,uber/packer,blairham/packer,feateds/packer,mansunkuo/packer,jae2/packer,Banno/packer,avldya/packer,umang1912/packer,monsterzz/packer,jamesleech/packer,monkeylittleinc/packer,qur/packer,mansunkuo/packer,uber/packer,feateds/packer,ldejager/packer,fnoeding/packer,dayglojesus/packer,jmptrader/packer,zhuzhih2017/packer,dragon788/packer,gbarberi/packer,ttaylor249/packer,ttaylor249/packer,ldejager/packer,sean-/packer,smaato/packer,ephemeralsnow/packer,tomwilkie/packer,btd/packer,timsutton/packer,marc-ta/packer,paulmey/packer,esemplare/packer,jsternberg/packer,mwhooker/packer,pvandervelde/packer,Akasurde/packer,bradparks/packer__create_vm_for_aws_docker_vagrant_etc,gbarberi/packer,danschaffer/packer,bhcleek/packer,rickard-von-essen/packer,youhong316/packer,jarl-dk/packer,styro/packer,threatstream/packer,arizvisa/packer,ober/packer,ricardclau/packer,c22/packer,markpeek/packer,qur/packer,shaunduncan/packer,timsutton/packer,higebu/packer,bensojona/packer,mkuzmin/packer,sneal/packer,stagrlee/packer,tracyde/packer,rnaveiras/packer,boumenot/packer,c22/packer,mohae/packer,israelshirk/packer,bhcleek/packer,jimmythedog/packer,justincampbell/packer,ChrisLundquist/packer,mitchellh/packer,tomwilkie/packer,gildas/packer-hyperv,Sheile/packer,samdunne/packer,burnzy/packer,zeliff/packer,gildas/packer-hyperv,yonglehou/packer,esemplare/packer,legal90/packer,kgrvamsi/packer,jmptrader/packer,dev-head/packer,orivej/packer,mindjiver/packer,youhong316/packer,markpeek/packer,markpeek/packer,jen20/packer,ricardclau/packer,samdunne/packer,kgrvamsi/packer,c22/packer,life360/packer,zakuro9715/packer,oliviertremblay/packer,sgirones/packer,vtolstov/packer,jimmythedog/packer,hazelesque/packer,BayanGroup/packer,mitchellh/packer,KohlsTechnology/packer,dave2/packer,eranchetz/packer,zakuro9715/packer,taliesins/packer,rk295/packer,p1-nico/packer,napramirez/packer,mkuzmin/packer,sequenceiq/packer,rickard-von-essen/packer,tas50/packer,DualSpark/packer,Xiol/packer,tsmolka/packer,grange74/packer,ricardclau/packer,styro/packer,mvanholsteijn/packer,mafrosis/packer,mindjiver/packer,sean-/packer,masbog/packer,beni55/packer,zhuzhih2017/packer,life360/packer,leroux/packer,esemplare/packer,quantang/packer,smaato/packer,routelastresort/packer,hashicorp/packer,dayglojesus/packer,umang1912/packer,sequenceiq/packer,shaunduncan/packer,hasJackb/packer,pvandervelde/packer,sequenceiq/packer,sobrinho/packer,vtolstov/packer,taliesins/packer,pieter-lazzaro/packer,smaato/packer,mvanholsteijn/packer,paulvollmer/packer,dieu/packer,tb3088/packer,jen20/packer,tas50/packer,icnocop/packer,LinusU/packer,pearkes/packer,hazelesque/packer,KFishner/packer,shaunduncan/packer,KohlsTechnology/packer,dragon788/packer,quantang/packer,gbarberi/packer,marc-ta/packer,visioncritical/packer,routelastresort/packer,jamesbishopp/packer,ricardclau/packer,jasonchaffee/packer,ChrisLundquist/packer,leroux/packer,pecigonzalo/packer,kpettijohn/packer,threatstream/packer,danschaffer/packer,israelshirk/packer,KohlsTechnology/packer,esemplare/packer,tb3088/packer,ephemeralsnow/packer,dantran/packer,hazelesque/packer,samdunne/packer,dayglojesus/packer,sansb/packer,ttaylor249/packer,sansb/packer,mayflower/packer,pieter-lazzaro/packer,jean/packer,tb3088/packer,noplay/packer,evertrue/packer,jarl-dk/packer,mafrosis/packer,evertrue/packer,sean-/packer,beni55/packer,life360/packer,shaunduncan/packer,pecigonzalo/packer,Sheile/packer,mwhooker/packer,dev-head/packer,napramirez/packer,henrysher/packer,zhuzhih2017/packer,quantang/packer,bhcleek/packer,bradparks/packer__create_vm_for_aws_docker_vagrant_etc,KohlsTechnology/packer,evertrue/packer,Silver90/packer,dave2/packer,atlassian/packer,styro/packer,arizvisa/packer,paulmey/packer,yonglehou/packer,trumant/packer,grange74/packer,quantang/packer,justincampbell/packer,sean-/packer,pvanbuijtene/packer,jean/packer,smaato/packer,masbog/packer,bryson/packer,aneeshusa/packer,jonathankarsh/packer,mitchellh/packer,c22/packer,mohae/packer,routelastresort/packer,samdunne/packer,mitchellh/packer,dragon788/packer,jae2/packer,jasonchaffee/packer,zhuzhih2017/packer,dave2/packer,rrva/packer,eranchetz/packer,rnaveiras/packer,LinusU/packer,mkuzmin/packer,bhcleek/packer,jen20/packer,burnzy/packer,israelshirk/packer,dayglojesus/packer,cyberhouse/packer,kgrvamsi/packer,rickard-von-essen/packer,taliesins/packer,ttaylor249/packer,pvandervelde/packer,ttaylor249/packer,p1-nico/packer,higebu/packer,pecigonzalo/packer,jonathankarsh/packer,mohae/packer,jsternberg/packer,mkuzmin/packer,feateds/packer,jszwedko/packer,sneal/packer,orivej/packer,mansunkuo/packer,jmptrader/packer,pieter-lazzaro/packer,stagrlee/packer,danschaffer/packer,uber/packer,masbog/packer,mindjiver/packer,LinusU/packer,ober/packer,mgk/packer,rnaveiras/packer,zakuro9715/packer,zeliff/packer,jimmythedog/packer,AlphaStaxLLC/packer,tomwilkie/packer,pearkes/packer,pvanbuijtene/packer,sneal/packer,orivej/packer,dieu/packer,kgrvamsi/packer,sansb/packer,Xiol/packer,rk295/packer,btd/packer,mayflower/packer,israelshirk/packer,mgk/packer,threatstream/packer,tsmolka/packer,gildas/packer-hyperv,grubernaut/packer,icnocop/packer,timsutton/packer,bradparks/packer__create_vm_for_aws_docker_vagrant_etc,dantran/packer,zeliff/packer,DualSpark/packer,jamesleech/packer,pvanbuijtene/packer,monsterzz/packer,vtolstov/packer,mayflower/packer,timsutton/packer,tb3088/packer,p1-nico/packer,taliesins/packer,dave2/packer,fnoeding/packer,boumenot/packer,paulmey/packer,leroux/packer,manicai/packer,styro/packer,grubernaut/packer,mvanholsteijn/packer,rickard-von-essen/packer,yonglehou/packer,rk295/packer,sgirones/packer,mohae/packer,jgkim/packer,atlassian/packer,paulmey/packer,monkeylittleinc/packer,ChrisLundquist/packer,jarl-dk/packer,Banno/packer,dieu/packer,bradparks/packer__create_vm_for_aws_docker_vagrant_etc,oliviertremblay/packer,dieu/packer,youhong316/packer,jen20/packer,fnoeding/packer,beni55/packer,visioncritical/packer,mgk/packer,marc-ta/packer,qur/packer,KohlsTechnology/packer,Cbeck527/packer,tracyde/packer,rnaveiras/packer,sequenceiq/packer,ober/packer,beni55/packer,taliesins/packer,rrva/packer,yoctocloud/packer,mwhooker/packer,masbog/packer,pieter-lazzaro/packer,AlphaStaxLLC/packer,jmptrader/packer,sansb/packer,danschaffer/packer,btd/packer,yoctocloud/packer,arizvisa/packer,leroux/packer,KohlsTechnology/packer,jean/packer,rrva/packer,cyberhouse/packer,sneal/packer,mefellows/packer,pearkes/packer,markpeek/packer,pvandervelde/packer,atlassian/packer,vtolstov/packer,lantrix/packer,avldya/packer,bryson/packer,paulmey/packer,mayflower/packer,fnoeding/packer,grange74/packer,Akasurde/packer,bensojona/packer,p1-nico/packer,lantrix/packer,jsternberg/packer,tsmolka/packer,mwhooker/packer,zakuro9715/packer,c22/packer,lantrix/packer,pieter-lazzaro/packer,rnaveiras/packer,Sheile/packer,mkuzmin/packer,KFishner/packer,tomwilkie/packer,markpeek/packer,mkuzmin/packer,jamesleech/packer,kpettijohn/packer,feateds/packer,icnocop/packer,burnzy/packer,jarl-dk/packer,Silver90/packer,sgirones/packer,eranchetz/packer,bensojona/packer,jamesbishopp/packer,mindjiver/packer,napramirez/packer,dantran/packer,mwhooker/packer,ober/packer,tas50/packer,noplay/packer,Sheile/packer,jszwedko/packer,routelastresort/packer,blairham/packer,aneeshusa/packer,bryson/packer,yoctocloud/packer,qur/packer,tas50/packer,ChrisLundquist/packer,bhcleek/packer,legal90/packer,jasonchaffee/packer,arizvisa/packer,jae2/packer,trumant/packer,oliviertremblay/packer,burnzy/packer,rnaveiras/packer,Akasurde/packer,threatstream/packer,grubernaut/packer,mansunkuo/packer,mgk/packer,ldejager/packer,pvanbuijtene/packer,paulvollmer/packer,BayanGroup/packer,Xiol/packer,Akasurde/packer,dev-head/packer,arizvisa/packer,yoctocloud/packer,stagrlee/packer,DualSpark/packer,justincampbell/packer,Silver90/packer,marc-ta/packer,pearkes/packer,napramirez/packer,rodm/packer,Cbeck527/packer,zeliff/packer,legal90/packer,mansunkuo/packer,monkeylittleinc/packer,cyberhouse/packer,oliviertremblay/packer,dave2/packer,ChrisLundquist/packer,jimmythedog/packer,eranchetz/packer,manicai/packer,higebu/packer,timsutton/packer,pecigonzalo/packer,hashicorp/packer,marklap/packer,trumant/packer,tb3088/packer,dayglojesus/packer,life360/packer,noplay/packer,threatstream/packer,higebu/packer,KFishner/packer,KFishner/packer,life360/packer,uber/packer,blairham/packer,paulvollmer/packer,jae2/packer,marklap/packer,taliesins/packer,Silver90/packer,hasJackb/packer,noplay/packer,markpeek/packer,p1-nico/packer,pvandervelde/packer,gildas/packer-hyperv,tracyde/packer,monsterzz/packer,grubernaut/packer,ober/packer,youhong316/packer,manicai/packer,grubernaut/packer,ricardclau/packer,henrysher/packer,sobrinho/packer,yoctocloud/packer,avldya/packer,jimmythedog/packer,AlphaStaxLLC/packer,BayanGroup/packer,tas50/packer,jgkim/packer,boumenot/packer,smaato/packer,kpettijohn/packer,sean-/packer,mafrosis/packer,paulvollmer/packer,jgkim/packer,youhong316/packer,avldya/packer,Cbeck527/packer,manicai/packer,vtolstov/packer,tsmolka/packer,tsmolka/packer,jszwedko/packer,rodm/packer,jsternberg/packer,jgkim/packer,jasonchaffee/packer,ephemeralsnow/packer,hasJackb/packer,tsmolka/packer,pvanbuijtene/packer,mafrosis/packer,p1-nico/packer,legal90/packer,ldejager/packer,umang1912/packer,zhuzhih2017/packer,BayanGroup/packer,henrysher/packer,smaato/packer,dev-head/packer,zhuzhih2017/packer,mitchellh/packer,jen20/packer,sobrinho/packer,gbarberi/packer,sobrinho/packer,monkeylittleinc/packer,bryson/packer,jasonchaffee/packer,Akasurde/packer,hashicorp/packer,boumenot/packer,hasJackb/packer,rk295/packer