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
Merge "SQL scripts should not manage transactions"