currently --db-dry-run works only if you use south.db.db in your migration files. If you modify the migration files to use some alternate database from south.db.dbs dict, dry run will not work and your changes will get applied to database. This commit fixes that by applying dry_run=True to all database objects, not only the default one, and then it works as-expected.
Oh and btw - using migrate --database some_db doesn't help if you have multiple databases you use. I for example have one database as default and other as actual data database... dry-run sticks only to default one.