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 "Use common db model class from Oslo"