Make quota rollback checks more robust in conductor tests

The existing cold migrate tests in conductor were mocking out the quota
object's rollback method but not actually validating that the quota
object is constructed properly or that the quota.QUOTAS.rollback static
method was called with the correct project_id and user_id given the
context and instance being used.  This patches makes the tests more
robust by not just making sure a method is called but that it's called
correctly.

Related-Bug: #1296502

Change-Id: Ib367d728b6bd50998db32d235f41212d0f94a8f7
Merge "Make quota rollback checks more robust in conductor tests"