Removes use of timeutils.set_time_override

The set_time_override function in timeutils was written as a
helper function to mock utcnow for unittests before 'mock' was
generally used. Now that we have mock and fixture, we no longer
need to use it.

Change-Id: I7c3793d7a442dc85aec405041c3c9ce6f7503bd8
Closes-Bug: #1266962
Merge "Removes use of timeutils.set_time_override"