Remove now-unnecessary client creation hacks

Clients that can use ksc Session don't need the old junk to
fake auth anymore:
* compute
* volume

Clients that still need to be fed credentials can pick directly
from the auth object in clientmanager.  The _token attribute is
removed, the token can be retrieved from the auth object:

  openstackclient/tests/common/test_clientmanager.py

This change will break any plugin that relies on getting a token
from instance._token. They should be updated to use the above, or
preferable, to use keystoneclient.session.Session to create its
HTTP interface object.

Change-Id: I877a29de97a42f85f12a14c274fc003e6fba5135
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