Add possibility to build AMIs in packer unsupported regions

Latest release of packer (1.2.4) doesn't still support ap-northeast-3 region.
Packer 1.1.1 doesn't support neither ap-northeast-3 nor eu-west-3.
Waiting for it, we set skip_region_validation to true. The default is false.

Ref:
https://www.packer.io/docs/builders/amazon-ebs.html#skip_region_validation

Signed-off-by: Enrico Usai <usai@amazon.com>

Author: Enrico Usai

amis/packer_alinux.json

@@ -27,6 +27,7 @@
       "ami_name" : "cfncluster-{{user `cfncluster_version`}}-amzn-hvm-{{user `build_date`}}",
       "subnet_id" : "{{user `subnet_id`}}",
       "vpc_id" : "{{user `vpc_id`}}",
+      "skip_region_validation" : true,
       "associate_public_ip_address" : true,
       "sriov_support" : true,
       "ena_support" : true,

amis/packer_centos6.json

@@ -27,6 +27,7 @@
       "ami_name" : "cfncluster-{{user `cfncluster_version`}}-centos6-hvm-{{user `build_date`}}",
       "subnet_id" : "{{user `subnet_id`}}",
       "vpc_id" : "{{user `vpc_id`}}",
+      "skip_region_validation" : true,
       "associate_public_ip_address" : true,
       "sriov_support" : true,
       "ena_support" : true,

amis/packer_centos7.json

@@ -27,6 +27,7 @@
       "ami_name" : "cfncluster-{{user `cfncluster_version`}}-centos7-hvm-{{user `build_date`}}",
       "subnet_id" : "{{user `subnet_id`}}",
       "vpc_id" : "{{user `vpc_id`}}",
+      "skip_region_validation" : true,
       "associate_public_ip_address" : true,
       "sriov_support" : true,
       "ena_support" : true,

amis/packer_ubuntu1404.json

@@ -27,6 +27,7 @@
       "ami_name" : "cfncluster-{{user `cfncluster_version`}}-ubuntu-1404-lts-hvm-{{user `build_date`}}",
       "subnet_id" : "{{user `subnet_id`}}",
       "vpc_id" : "{{user `vpc_id`}}",
+      "skip_region_validation" : true,
       "associate_public_ip_address" : true,
       "sriov_support" : true,
       "ena_support" : true,

amis/packer_ubuntu1604.json

@@ -27,6 +27,7 @@
       "ami_name" : "cfncluster-{{user `cfncluster_version`}}-ubuntu-1604-lts-hvm-{{user `build_date`}}",
       "subnet_id" : "{{user `subnet_id`}}",
       "vpc_id" : "{{user `vpc_id`}}",
+      "skip_region_validation" : true,
       "associate_public_ip_address" : true,
       "sriov_support" : true,
       "ena_support" : true,