support client certificate file
support client certificate file

When AUTH_URL and each endpoints need SSL client authentication, we
have to specify cert file and private key file.
So, add optional config, cert and key.

dave2/packer,ChrisLundquist/packer,grubernaut/packer,atlassian/packer,zhuzhih2017/packer,sean-/packer,routelastresort/packer,paulmey/packer,tb3088/packer,rnaveiras/packer,c22/packer,hashicorp/packer,KohlsTechnology/packer,smaato/packer,tas50/packer,tb3088/packer,hashicorp/packer,ricardclau/packer,youhong316/packer,youhong316/packer,p1-nico/packer,ricardclau/packer,pvandervelde/packer,bryson/packer,tas50/packer,tsmolka/packer,mitchellh/packer,tb3088/packer,KohlsTechnology/packer,tb3088/packer,atlassian/packer,p1-nico/packer,tb3088/packer,atlassian/packer,pvandervelde/packer,arizvisa/packer,grubernaut/packer,markpeek/packer,mkuzmin/packer,rnaveiras/packer,zhuzhih2017/packer,jen20/packer,pvandervelde/packer,rnaveiras/packer,atlassian/packer,c22/packer,sean-/packer,KohlsTechnology/packer,paulmey/packer,rnaveiras/packer,boumenot/packer,zhuzhih2017/packer,p1-nico/packer,ChrisLundquist/packer,rnaveiras/packer,boumenot/packer,pieter-lazzaro/packer,timsutton/packer,KohlsTechnology/packer,jen20/packer,bryson/packer,ricardclau/packer,mkuzmin/packer,arizvisa/packer,KohlsTechnology/packer,pecigonzalo/packer,smaato/packer,mkuzmin/packer,mkuzmin/packer,jen20/packer,timsutton/packer,sean-/packer,jen20/packer,atlassian/packer,jen20/packer,ricardclau/packer,pvandervelde/packer,pieter-lazzaro/packer,tas50/packer,arizvisa/packer,tsmolka/packer,mitchellh/packer,grubernaut/packer,bryson/packer,smaato/packer,smaato/packer,pvandervelde/packer,smaato/packer,p1-nico/packer,mkuzmin/packer,routelastresort/packer,arizvisa/packer,ChrisLundquist/packer,rnaveiras/packer,routelastresort/packer,c22/packer,paulmey/packer,timsutton/packer,markpeek/packer,paulmey/packer,markpeek/packer,ricardclau/packer,sean-/packer,bryson/packer,zhuzhih2017/packer,ChrisLundquist/packer,bryson/packer,mkuzmin/packer,youhong316/packer,taliesins/packer,dave2/packer,mitchellh/packer,tas50/packer,tas50/packer,paulmey/packer,taliesins/packer,youhong316/packer,p1-nico/packer,tas50/packer,sean-/packer,timsutton/packer,atlassian/packer,pecigonzalo/packer,markpeek/packer,boumenot/packer,dave2/packer,pvandervelde/packer,jen20/packer,p1-nico/packer,pecigonzalo/packer,tsmolka/packer,ChrisLundquist/packer,grubernaut/packer,dave2/packer,tsmolka/packer,tsmolka/packer,c22/packer,routelastresort/packer,taliesins/packer,markpeek/packer,zhuzhih2017/packer,routelastresort/packer,timsutton/packer,markpeek/packer,grubernaut/packer,timsutton/packer,bryson/packer,KohlsTechnology/packer,taliesins/packer,arizvisa/packer,zhuzhih2017/packer,dave2/packer,boumenot/packer,youhong316/packer,pecigonzalo/packer,taliesins/packer,pieter-lazzaro/packer,c22/packer,paulmey/packer,c22/packer,pieter-lazzaro/packer,grubernaut/packer,hashicorp/packer,pieter-lazzaro/packer,taliesins/packer,dave2/packer,mitchellh/packer,sean-/packer,ricardclau/packer,tb3088/packer,mitchellh/packer,ChrisLundquist/packer,boumenot/packer,tsmolka/packer,pieter-lazzaro/packer,youhong316/packer,smaato/packer,hashicorp/packer,arizvisa/packer