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 patch, nova-manage db sync succeeds using mysqlconnector.

Change-Id: I57e6ecdafe90110eaffe757f0ef0bf7b41b0f3e3
Merge "Issue multiple SQL statements in separate engine.execute() calls"