build: add timestamps to start/end of build
build: add timestamps to start/end of build

During recent CI issues I got an idea that it would be nice to have
information how much time building specific image took. On local systems
we can do log and then check 'modified_time - creation_time' but not on
CI jobs.

Fresh image:

INFO:kolla.common.utils.base:Building started at 2019-06-26 13:53:08.393544
INFO:kolla.common.utils.base:Ended at 2019-06-26 13:56:40.354761 (took 0:03:31.961225)

Rebuild:

INFO:kolla.common.utils.base:Building started at 2019-06-26 14:00:34.913194
INFO:kolla.common.utils.base:Built at 2019-06-26 14:00:38.988874 (took 0:00:04.075686)

Failed images do not get second line due to logger.exception() being
used.

Change-Id: Id13545224fa0fbb729d46386a10505e0989fc3c6

stackforge/kolla,openstack/kolla,rahulunair/kolla,stackforge/kolla,stackforge/kolla,rahulunair/kolla,openstack/kolla