Use InnoDB storage engine when creating tables in migrations

Although __table_args__ is set correctly in the base model, existing
migration scripts were not generated with the mysql_engine option in
the alembic output, likely due to:

https://bitbucket.org/zzzeek/alembic/issue/110/

This adds the mysql_engine option to each table creation operation.

Change-Id: I990e8d64bcadf2be4e1e319319bc23225123b304
Closes-Bug: #1288358