Stub out unmocked keystone in test_engine_service

Keystone needs to be mocked out whenever stacks may do something that
triggers authentication. In this case failures to auth would not result
in a failure of the test because we are not asserting the end state of
the stack or the list of events we expect.

Change-Id: I401971cfc984bd71c611eae70aeb33b2f5a3c8c6
Closes-Bug: #1227962
Merge "Stub out unmocked keystone in test_engine_service"