add celery backend config

... so that we can wait on tasks

inventory_provider/tasks/config.py

@@ -3,3 +3,6 @@ from os import getenv
 broker_url = getenv(
     'CELERY_BROKER_URL',
     default='redis://test-dashboard02.geant.org:6379/1')
+result_backend = getenv(
+    'CELERY_BROKER_URL',
+    default='redis://test-dashboard02.geant.org:6379/1')