Make raw_vars compatible with play vars and Ansible 2.1

[fullyint]

discourse.roots.io/t/7861/3 revealed that raw_vars auto-escaping stopped working as of Ansible 2.1. Ansible made an internal API change removing the possibility for vars plugins to access and modify vars for Host objects. The fate of Ansible vars plugins appears uncertain, given that Ansible...

was planning on killing vars_plugins in their current form --ref

This PR moves plugins/vars/vars.py to plugins/callback/vars.py. In the new callback context, the plugin can still access and modify Host vars, restoring the raw_vars auto-escaping functionality.

Given that this new callback context also grants access to play vars, the raw_vars can now escape play vars (i.e., vars defined in playbooks) in addition to group vars. Play vars take precedence.

I moved the Ansible 1.x warning to the callback plugin output.py. As the designated stdout callback plugin, output.py is processed first, before vars.py.

[swalkinshaw]

Annoying 😔 but changes look good