SQL scripts should not manage transactions

SQL scripts should not manage transactions

Cinder SQL scripts assume too much about the environment and the way in
which they are executed by sqlalchemy-migrate. Particularly, they manage
transactions on their own even though sqlalchemy-migrate do it itself,
plus it assumes that scripts are executed in one go (even though
standard Python DB-API 2.0 does not mention multi-statement SQL commands
to be executed on DB cursors.

blueprint enable-mysql-connector

Change-Id: I10c58b3af75d3ab9153a8bbd2a539bf1577de328
Closes-Bug: 1368391