api/ec2: implement describe_instance_attribute()
api/ec2: implement describe_instance_attribute()

This patch implements DescribeInstanceAttribute.

NoBodyCam/TftpPxeBootBareMetal,klmitch/nova,savi-dev/nova,virtualopensystems/nova,mahak/nova,CloudServer/nova,yrobla/nova,redhat-openstack/nova,citrix-openstack-build/nova,TieWei/nova,Juniper/nova,zaina/nova,iuliat/nova,SUSE-Cloud/nova,zzicewind/nova,sileht/deb-openstack-nova,ntt-sic/nova,affo/nova,maoy/zknova,gooddata/openstack-nova,dawnpower/nova,berrange/nova,whitepages/nova,CiscoSystems/nova,luogangyi/bcec-nova,CiscoSystems/nova,mahak/nova,rickerc/nova_audit,Triv90/Nova,russellb/nova,paulmathews/nova,berrange/nova,Francis-Liu/animated-broccoli,DirectXMan12/nova-hacking,thomasem/nova,salv-orlando/MyRepo,shail2810/nova,spring-week-topos/nova-week,bclau/nova,eayunstack/nova,ewindisch/nova,scripnichenko/nova,mandeepdhami/nova,vmturbo/nova,Stavitsky/nova,gspilio/nova,nikesh-mahalka/nova,devendermishrajio/nova_test_latest,BeyondTheClouds/nova,LoHChina/nova,CEG-FYP-OpenStack/scheduler,badock/nova,paulmathews/nova,gooddata/openstack-nova,alvarolopez/nova,shootstar/novatest,MountainWei/nova,belmiromoreira/nova,sridevikoushik31/nova,redhat-openstack/nova,Metaswitch/calico-nova,rajalokan/nova,leilihh/nova,sileht/deb-openstack-nova,cloudbase/nova,KarimAllah/nova,akash1808/nova,rrader/nova-docker-plugin,mgagne/nova,cernops/nova,tanglei528/nova,alaski/nova,cloudbase/nova-virtualbox,alexandrucoman/vbox-nova-driver,tangfeixiong/nova,JianyuWang/nova,sebrandon1/nova,vmturbo/nova,whitepages/nova,Juniper/nova,dawnpower/nova,zzicewind/nova,virtualopensystems/nova,NewpTone/stacklab-nova,savi-dev/nova,salv-orlando/MyRepo,dstroppa/openstack-smartos-nova-grizzly,yatinkumbhare/openstack-nova,TwinkleChawla/nova,edulramirez/nova,shahar-stratoscale/nova,yosshy/nova,mahak/nova,kimjaejoong/nova,zhimin711/nova,double12gzh/nova,scripnichenko/nova,sebrandon1/nova,fnordahl/nova,cloudbase/nova,maheshp/novatest,edulramirez/nova,orbitfp7/nova,akash1808/nova,imsplitbit/nova,Tehsmash/nova,vladikr/nova_drafts,eonpatapon/nova,gooddata/openstack-nova,CCI-MOC/nova,Yuriy-Leonov/nova,mmnelemane/nova,saleemjaveds/https-github.com-openstack-nova,devoid/nova,maheshp/novatest,cernops/nova,cloudbau/nova,leilihh/novaha,raildo/nova,takeshineshiro/nova,usc-isi/extra-specs,noironetworks/nova,blueboxgroup/nova,hanlind/nova,sileht/deb-openstack-nova,bgxavier/nova,Brocade-OpenSource/OpenStack-DNRM-Nova,leilihh/novaha,JioCloud/nova,belmiromoreira/nova,usc-isi/extra-specs,KarimAllah/nova,luogangyi/bcec-nova,isyippee/nova,bclau/nova,jianghuaw/nova,openstack/nova,josephsuh/extra-specs,yosshy/nova,ted-gould/nova,NewpTone/stacklab-nova,tealover/nova,takeshineshiro/nova,sacharya/nova,barnsnake351/nova,mikalstill/nova,noironetworks/nova,KarimAllah/nova,tudorvio/nova,gspilio/nova,thomasem/nova,watonyweng/nova,dims/nova,savi-dev/nova,psiwczak/openstack,eharney/nova,rajalokan/nova,rajalokan/nova,yatinkumbhare/openstack-nova,Triv90/Nova,OpenAcademy-OpenStack/nova-scheduler,salv-orlando/MyRepo,tealover/nova,usc-isi/nova,rrader/nova-docker-plugin,josephsuh/extra-specs,JioCloud/nova,ruslanloman/nova,NewpTone/stacklab-nova,yrobla/nova,viggates/nova,fajoy/nova,openstack/nova,ted-gould/nova,fajoy/nova,sridevikoushik31/openstack,houshengbo/nova_vmware_compute_driver,gspilio/nova,cyx1231st/nova,tianweizhang/nova,sridevikoushik31/nova,Yusuke1987/openstack_template,fnordahl/nova,silenceli/nova,CloudServer/nova,bigswitch/nova,eneabio/nova,phenoxim/nova,jianghuaw/nova,Francis-Liu/animated-broccoli,joker946/nova,russellb/nova,Juniper/nova,vmturbo/nova,spring-week-topos/nova-week,klmitch/nova,nikesh-mahalka/nova,eharney/nova,sridevikoushik31/nova,joker946/nova,maheshp/novatest,dstroppa/openstack-smartos-nova-grizzly,LoHChina/nova,klmitch/nova,TieWei/nova,petrutlucian94/nova_dev,saleemjaveds/https-github.com-openstack-nova,TwinkleChawla/nova,kimjaejoong/nova,apporc/nova,Yusuke1987/openstack_template,zaina/nova,angdraug/nova,shootstar/novatest,rajalokan/nova,usc-isi/nova,yrobla/nova,vladikr/nova_drafts,adelina-t/nova,jeffrey4l/nova,mmnelemane/nova,leilihh/nova,jeffrey4l/nova,sacharya/nova,hanlind/nova,eneabio/nova,BeyondTheClouds/nova,Juniper/nova,Tehsmash/nova,cloudbase/nova,usc-isi/nova,ewindisch/nova,ntt-sic/nova,devendermishrajio/nova,maoy/zknova,rahulunair/nova,mgagne/nova,felixma/nova,projectcalico/calico-nova,barnsnake351/nova,devendermishrajio/nova,sebrandon1/nova,hanlind/nova,fajoy/nova,petrutlucian94/nova,double12gzh/nova,petrutlucian94/nova_dev,klmitch/nova,iuliat/nova,Yuriy-Leonov/nova,alaski/nova,zhimin711/nova,akash1808/nova_test_latest,adelina-t/nova,felixma/nova,alexandrucoman/vbox-nova-driver,NeCTAR-RC/nova,rahulunair/nova,aristanetworks/arista-ovs-nova,DirectXMan12/nova-hacking,maelnor/nova,BeyondTheClouds/nova,akash1808/nova_test_latest,plumgrid/plumgrid-nova,russellb/nova,viggates/nova,cloudbau/nova,aristanetworks/arista-ovs-nova,orbitfp7/nova,psiwczak/openstack,dims/nova,sridevikoushik31/openstack,shail2810/nova,qwefi/nova,phenoxim/nova,mandeepdhami/nova,gooddata/openstack-nova,sridevikoushik31/openstack,maelnor/nova,apporc/nova,Metaswitch/calico-nova,dstroppa/openstack-smartos-nova-grizzly,cloudbase/nova-virtualbox,ruslanloman/nova,tudorvio/nova,affo/nova,shahar-stratoscale/nova,SUSE-Cloud/nova,varunarya10/nova_test_latest,j-carpentier/nova,MountainWei/nova,eneabio/nova,maoy/zknova,OpenAcademy-OpenStack/nova-scheduler,usc-isi/extra-specs,NoBodyCam/TftpPxeBootBareMetal,CEG-FYP-OpenStack/scheduler,devoid/nova,eonpatapon/nova,j-carpentier/nova,alvarolopez/nova,mikalstill/nova,Triv90/Nova,openstack/nova,jianghuaw/nova,jianghuaw/nova,JioCloud/nova_test_latest,isyippee/nova,imsplitbit/nova,badock/nova,NoBodyCam/TftpPxeBootBareMetal,bigswitch/nova,plumgrid/plumgrid-nova,sridevikoushik31/nova,silenceli/nova,blueboxgroup/nova,eayunstack/nova,houshengbo/nova_vmware_compute_driver,rahulunair/nova,psiwczak/openstack,tianweizhang/nova,cyx1231st/nova,mikalstill/nova,Brocade-OpenSource/OpenStack-DNRM-Nova,vmturbo/nova,JianyuWang/nova,petrutlucian94/nova,aristanetworks/arista-ovs-nova,raildo/nova,cernops/nova,projectcalico/calico-nova,paulmathews/nova,CCI-MOC/nova,citrix-openstack-build/nova,devendermishrajio/nova_test_latest,Stavitsky/nova,JioCloud/nova_test_latest,rickerc/nova_audit,tanglei528/nova,houshengbo/nova_vmware_compute_driver,NeCTAR-RC/nova,DirectXMan12/nova-hacking,watonyweng/nova,bgxavier/nova,varunarya10/nova_test_latest,angdraug/nova,qwefi/nova,josephsuh/extra-specs,tangfeixiong/nova