Revert update of base AMI for Ubuntu 16.04

The latest Ubuntu 16.04 AMI uses an aws-specific kernel.  Which
is probably awesome in many ways, but doesn't include a
linux-image-extras package or the bits needed to build the
nVidia kernel modules.  Need to sort that out later, but
for now, just use the older base image.

Signed-off-by: Brian Barrett <bbarrett@amazon.com>

Author: bwbarrett

packer_ubuntu1604.json

@@ -19,7 +19,7 @@
       "region" : "us-east-1",
       "ami_regions" : "{{user `build_for`}}",
       "ami_groups" : "{{user `ami_perms`}}",
-      "source_ami" : "ami-80861296",
+      "source_ami" : "ami-45c18152",
       "instance_type" : "{{user `instance_type`}}",
       "ssh_username" : "ubuntu",
       "ssh_pty" : true,