Sync oslo-incubator db.sqlalchemy b9e2499

oslo-incubator db.sqlalchemy has a change that's needed to
support SQLAlchemy 0.9.x. This syncs the change from
oslo-incubator and any other changes that have been made
since the last sync. The oslo-incubator commit hash is
b9e249962128b72a118252216eb4f0bcafd46da0

Here's the list of changes:

9fed4ed Fix Keystone doc build errors with SQLAlchemy 0.9
5b7e61c Dispose db connections pool on disconnect
d1988b9 Set sql_mode callback on connect instead of checkout

Do this to recreate:

 $ python update.py --nodeps --base keystone \
     --dest-dir ../keystone --modules db.sqlalchemy

Change-Id: I93b310d30fc471f52487087597c4d9974a0d722c
Partial-Bug: #1296333
Merge "Sync oslo-incubator db.sqlalchemy b9e2499"