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