Fixes #403 (Github Issue).

This keeps the roles cleaner and more independent of matrix-base,
which may be important for people building their own playbook
out of the individual roles and not using the matrix-base role.

# Auth server config
        auth:
            # Publicly accessible base URL for the login endpoints.
            # The prefix below is not implicitly added. This URL and all subpaths should be proxied
            # or otherwise pointed to the appservice's webserver to the path specified below (prefix).
            # This path should usually include a trailing slash.
            public: http://example.com/login/
            # Internal prefix in the appservice web server for the login endpoints.
            prefix: /login