Use common db model class from Oslo

Modified DictBase class - added common `models.ModelBase` class as
base class instead of object and removed duplicated code.
Added new oslo.config version to requirements.

blueprint use-common-oslo-db-code

Change-Id: Ib1b8883d878a1c95d04dfab05bfe87ce37d0d6cb
Merge "Use common db model class from Oslo"