Sync common db code from Oslo

This sync brings the latest openstack/common/db code from
olso-incubator until commit
9fed4ed Fix Keystone doc build errors with SQLAlchemy 0.9

Changes in  Rally caused by API changes in openstack.common.db code -
oslo.db no longer stores SQLAlchemy Engine and sessionmaker instances
globally and it's up to applications to create them. So we should add
methods for work with engine and session to Rally code.

Config database options was modified, so rally.conf.sample was updated.

Change-Id: Ia00f2c5fb54bb86f88440233f9bf79eab7b07589