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: I057d4bc3a8b1d4e96e6830cd3071dc96a05496dc
Partial-Bug: #1266962
Replace tearDown with addCleanup

Infra team has indicated that tearDown should not be used and should
be replaced with addCleanup in all places.

This patch replaces tearDown with addCleanup methods in cinder/tests/api.

Implements blueprint replace-teardown-with-addcleanup

Change-Id: I640c1985bf7fce6f2fbd4de8e56448b6c9af51ca
Merge "Removes use of timeutils.set_time_override"
Merge "Replace tearDown with addCleanup"