Move sql.Base.transaction

With the change to use oslo's db.sqlalchemy.sessions, the
sql.Base class isn't doing enough to justify its continued
existence.

This change is moving the transaction member out of Base.

Part of bp use-common-oslo-db-code

Change-Id: Ie0df7b397f8508e1c9be70ce9bbb2d733bde03ea
Merge "Move sql.Base.transaction"