Pin ubuntu1604 version

Signed-off-by: Sean Smith <seaam@amazon.com>

Author: Sean Smith

amis/packer_ubuntu1604.json

@@ -23,7 +23,7 @@
       "source_ami_filter": {
         "filters": {
           "virtualization-type": "hvm",
-          "name": "ubuntu/images/hvm-ssd/ubuntu-xenial-16.04-amd64-server-*",
+          "name": "ubuntu/images/hvm-ssd/ubuntu-xenial-16.04-amd64-server-20171121.1",
           "root-device-type": "ebs"
         },
         "owners": ["099720109477", "513442679011"],