Update NovaBase model per changes on oslo.db.sqlalchemy

Fixed NovaBase to inherit from models.TimestampMixin
Added metadata attribute
Added db.sqlalchemy to openstack-common.conf

Oslo sync command:
python update.py --modules db.sqlalchemy --base nova --dest-dir ../nova

Change-Id: Ie4ad8e8b235e146ddb946cdf7f15d74e060ee4a3
Merge "Update NovaBase model per changes on oslo.db.sqlalchemy"