Remove ClientManager._service_catalog

Anything that needs a service catalog can get it directly from
auth_ref.service_catalog, no need to carry the extra attribute.

ClientManager.get_endpoint_for_service_type() reamins the proper
method to get an endpoint for clients that still need one directly.

Change-Id: I809091c9c71d08f29606d7fd8b500898ff2cb8ae