sqlmigrate wraps it's outpout in BEGIN/COMMIT even if the database doesn't support transactional DDL

[Simon Charette]

The migration executor only adds the outer BEGIN/COMMIT ​if the migration is atomic and ​the schema editor can rollback DDL but the current sqlmigrate logic only takes migration.atomic into consideration.

The issue can be addressed by

1. Changing sqlmigrate ​assignment of self.output_transaction to consider connection.features.can_rollback_ddl as well.
2. Adding a test in tests/migrations/test_commands.py based on ​an existing test for non-atomic migrations that mocks connection.features.can_rollback_ddl to False instead of overdidding MIGRATION_MODULES to point to a non-atomic migration.

I marked the ticket as easy picking because I included the above guidelines but feel free to uncheck it if you deem it inappropriate.