Add tests for glance/notifier/notify_kombu

Add tests for glance/notifier/notify_kombu in
RabbitStrategyTestCase class.
Tests for:
- _close()
- _connect()
- time.sleep() in reconnect()
- processing KombuMaxRetriesReached error in _notify()

Change-Id: I39805ce460ce2b71dcb05e63cb898c5569d726b0
Use common db model class from Oslo

Modified common model class - added common `models.ModelBase` and
`models.TimestampMixin` classes as base class instead of object.
Renamed base model class from `ModelBase` to `GlanceBase`.
Removed duplicated code.

blueprint db-use-oslo-common-code

Change-Id: I6038adcfcb6b985736ac8d9374ac0a2fe1cdcc96
Merge "Add tests for glance/notifier/notify_kombu"
Merge "Use common db model class from Oslo"