add Ansible contrib layer

[BrianHicks]

After discussion on #526, here's an Ansible layer incorporating ansible and ansible-doc, loaded conditionally if the file matches one of:

• roles/**/*.yml
• group_vars/*
• host_vars/*
• site.yml (this could be removed, I took the "standard" looking paths from the recommended Ansible project structure but this one was a little more arbitrary than the first three.)

[syl20bnr]

Yeah ! Nice layer. 👍
I will probably create a category for provisioning and start moving this layer and the puppet one there.

Cherry-picked into develop, you can safely delete your branch.