Add official support (and update acceptance tests) for Ubuntu 16.04

[wyardley]

• [semi-related] Fix some issues with local spec tests Update 'require' statements and mock types to fix spec tests #620
• fix fixtures Update fixtures to voxpupuli/archive (from puppet-community) #619
• Add packages_ensure for python BREAKING: Ensure python package (adds manage_python option) #623
• Switch to camptocamp systemd module (if possible - may need to work around test failures and do this later)
• Update metadata and acceptance tests Add official support for Ubuntu 16.04 #624

[wyardley]

I'm going to table camptocamp systemd module for now; it adds an additional dependency, and yet doesn't yet support managing file limits the way we need to (it doesn't support unlimited, as best I can tell).