engine.clients always use keystoneclient auth_token

Always get the token from keystoneclient, even if there is one in
the request context, as if the context contains a username and
password, there is the possibility the keystoneclient auth_token
may be a reissued token which should be preferred to the one in
the context which may have expired.

Change-Id: Icbce9ae7a8ff7cad5eadec3de4f69b977949c265
Partial-Bug: #1306294
Merge "engine.clients always use keystoneclient auth_token"