Issue multiple SQL statements in separate engine.execute() calls

Some sqlalchemy drivers (eg: mysqlconnector) don't support
engine.execute("sql stmt 1; sql stmt 2;") and require a separate
execute() call for each SQL statement.  After discussions with
sqlalchemy author, he confirmed it would be better to fix callers rather
than attempt to patch in support for multiple statements.

With this fix, keystone-manage db_sync succeeds using mysqlconnector.

Change-Id: I658a4dd479dbdcd07345c97d810c85aada3fa35b
Merge "Issue multiple SQL statements in separate engine.execute() calls"