[AIRFLOW-2979] Make celery_result_backend conf Backwards compatible (apache#3832)

(apache#2806) Renamed `celery_result_backend` to `result_backend` and broke backwards compatibility.

Author: kaxil

airflow/configuration.py

@@ -139,6 +139,7 @@ class AirflowConfigParser(ConfigParser):
         'celery': {
             # Remove these keys in Airflow 1.11
             'worker_concurrency': 'celeryd_concurrency',
+            'result_backend': 'celery_result_backend',
             'broker_url': 'celery_broker_url',
             'ssl_active': 'celery_ssl_active',
             'ssl_cert': 'celery_ssl_cert',