Add convenience function to allow easy stubbing of keystoneclient
in all tests where explicit control and verification is not required.
This is a precursor to making all clients use the auth_token property
from keystoneclient (instead of using the context auth_token in
preference) such that trust tokens can be refreshed before expiry.
Partial-Bug: #1306294
Change-Id: I9ba4595b8750ff769e76972cc30b55a68253e76d
f9d9fae233