Handle URLs via the session and auth_plugins

In the future clients will simply pass the service they expect to talk
to and the path. This will prevent every service trying to get their own
base urls from the service catalog individually.

This can later be extended to have the auth plugin actually contact the
URL from the service catalog which will let us have unversioned
endpoints in the catalog handled from a single location.

Change-Id: I80f0b5b1dbb45565fec09d1cb2c0552cfb9a72f5
blueprint: auth-plugin-endpoints
Merge "Handle URLs via the session and auth_plugins"