db: Make EngineFacade a Facade

This blueprint proposes a new declarative
system of connection management and
session and transaction scoping for
all projects that currently make use
of oslo.db.session.EngineFacade.

Change-Id: I8c0efa3b859c2fe5f06f4c0416c5323362d7ef72
Merge "db: Make EngineFacade a Facade"