Provide a way to proxy calls to a plugin

Provide a generic means of wrapping calls to an existing plugin. This would allow a plugin to generically modify the responses of an existing plugin without the need to subclass it and target a specific plugin

Immediate use cases:
- A plugin that ignores the catalog and returns only a specific endpoint for a service_type. We want this to be a proxy because otherwise we'd need to provide a subclass for every auth plugin type.

- A plugin that always uses the same session. Plugins accept a session object so that they can do things like authenticate. This is generally fine but if you are mixing and matching sessions then you may need the plugin to only authenticate using a specific session. A wrapper could ensure that the session passed to the plugin is always a predefined one instead of the session for the current request.

No-Spec approval: http://eavesdrop.openstack.org/meetings/keystone/2015/keystone.2015-03-24-18.03.log.html