(Docs) Unify OS version info and plug Puppet Enterprise in more places

- Now that we're shipping a PE with MCollective 2.x, we can recommend it more
  vigorously in the 2.x deployment pages.
- To make things easier for the release team, we moved the OS versions supported
  by our repositories into some central files, in the source/_includes directory
  of the puppet-docs repo. Mco docs can take advantage of this too.