Fix Migrations.all_migrations() list by clearing cache

Creating Tenant using `TenantMixin` fails. `TenantMixin.create_schema` use `migrate_schemas` command. Command doesn't clear `Migrations` cache so `settings.SOUTH_MIGRATION_MODULES` filled with `'ignore'` values is ingnored by south Migrations. This cause use on tenant schema migrations also from shared schema.
Fix just clear south `Migrations` cache after changing `settings.SOUTH_MIGRATION_MODULES`.
Probably it can be done better, but this solve issue, and I think that place is intuitive.