The basic Auth plugin for v3 tokens makes the assumption that you need
to pass in some AuthMethod objects. This works well for most auth types
where you want the plugin to construct the auth request for you.
In the case of federation though we want to be able to have a rescoping
plugin that will return an auth_ref and not take any auth_methods as
arguments.
Extract the most basic part of the Auth plugin into BaseAuth class that
Auth and federation plugins can both inherit from.
Change-Id: Ia8c8c614b8eb51170346ff5b1e20a1e7ebbb47de
fc1f5a7963