Use timestamp in loopingcall

All time related data in loopingcall are in seconds.
Therefore creating datetime objects only adds overhead to the module.
This patch replaces all datetime objects with utc timestamps in loopingcall.

The rational of using timeutils was disscussed here,
https://review.openstack.org/#/c/16310/1..5/openstack/common/loopingcall.py,cm
which was to use set_time_override to avoid actual time delays in test scripts.
Now that set_time_override is deprecated, we no longer need to use timeutils.

A lambda function returning time.time() was declared due to mock collisions
with time.time() called in the logging module.
Changes to test scripts were made accordingly.

Change-Id: I38749f3c7bfff23164481b2341d56154ba737765
Merge "Use timestamp in loopingcall"