Make use of 'current' apps so RBAC ImplicitRoleField can work during migrations

While a migration is taking place, we can't juse use normal model
references like Role and RolePermission, nor can we use generic foreign
keys without manually referring to the content type and object id
fields.

awx/main/migrations/_rbac.py

@@ -5,7 +5,7 @@ from django.db.models import Q
 from django.utils.timezone import now
 
 from collections import defaultdict
-from awx.main.utils import getattrd
+from awx.main.utils import getattrd, set_current_apps
 
 import _old_access as old_access
 logger = logging.getLogger(__name__)
@@ -26,6 +26,10 @@ def log_migration(wrapped):
         return wrapped(*args, **kwargs)
     return wrapper
 
+@log_migration
+def init_rbac_migration(apps, schema_editor):
+    set_current_apps(apps)
+
 @log_migration
 def migrate_users(apps, schema_editor):
     User = apps.get_model('auth', "User")