Fix incompatibilities in tests with keystoneclient 0.6

The patch removes usage of StubOutClassWithMocks with keystone Client
class, which doesn't work anymore now that there is an ABC in the MRO.

Closes-Bug: #1279907
(cherry picked from commit 26fe7a961f644e1c93a92c8a73b25d649137f7b9)

Conflicts:
	heat/tests/test_heatclient.py

Change-Id: I4e8ff2cde2723060faf9906ecf51b13911ac5f42