Add a new arg just for git refs.

This seperates the git refs from the rest of the vars because the git refs should end up in
the ami document in mongo but the other vars should not.
Add an env tag to studio and edxapp dns names.
Allow studio access for urls with env prefixes.
Run migrations before syncing the DB.
Begin referencing environment and deployment instead of env_type and env_name.
Have only one YAML message header.
Merge pull request #745 from edx/feanil/proper_extra_vars

Feanil/proper extra vars