WIP - Add Ubuntu 16.04 support

[mzia]

Added Ubuntu 16.04 support

[konklone]

Heck yeah! @jgrevich @mzia The commit and PR says this is a WIP -- what's left to do?

[jgrevich]

• Move parts of packer config (region, source_ami, instance_type, ssh_username, etc) to configurable vars with defaults
• There's some issue with the 16.04 build in that the AMI builds but you cannot ssh in afterwards. I haven't had time to look into it yet.

Perhaps I'm missing something regarding the SSH issue.

[alain-hoang]

In Ubuntu 16.04 a bug related to ssh login failures can ben found at https://bugs.launchpad.net/ubuntu/xenial/+source/sssd/+bug/1579092 which could be related to the SSH issue.

[joelchen]

Any updates for Ubuntu 16.04 support?

[b-long]

I'm also interested in Ubuntu 16.04 support and I'd be happy to help test.

[mzia]

@jgrevich are we in need to do this any more for this
https://github.com/18F/identity-base-image ?