Remove sqlalchemy calling back to DB API

sqlalchemy unnecessarily calls back to DB API in a few spots.  It's
more efficient to not do so.

Also, when dbapi_use_tpool is enabled, this causes a lock up due to an
eventlet bug.

Fixes bug 1154811

Change-Id: I5937a2dce85a09502f40ebcd99afb441938c1289
Merge "Remove sqlalchemy calling back to DB API"