title: Apache Airflow celery executor is not getting result backend
body: <p>I am running Apache Airflow version 1.9.0 and when I try to run a task from UI, I get the following error in airflow scheduler console:</p>
  
  <pre><code>[2018-05-08 12:09:06,737] {jobs.py:1077} INFO - No tasks to consider for execution.
  [2018-05-08 12:09:06,738] {jobs.py:1662} INFO - Heartbeating the executor
  [2018-05-08 12:09:06,738] {celery_executor.py:101} ERROR - Error syncing the celery executor, ignoring it:
  [2018-05-08 12:09:06,738] {celery_executor.py:102} ERROR - No result backend configured.  Please see the documentation for more information.
  Traceback (most recent call last):
    File "/usr/local/lib/python2.7/dist-packages/airflow/executors/celery_executor.py", line 83, in sync
      state = async.state
    File "/usr/local/lib/python2.7/dist-packages/celery/result.py", line 329, in state
      return self.backend.get_status(self.id)
    File "/usr/local/lib/python2.7/dist-packages/celery/backends/base.py", line 547, in _is_disabled
      'No result backend configured.  '
  NotImplementedError: No result backend configured.  Please see the documentation for more information.
  </code></pre>
  
  <p>In my airflow.cfg, I have the following variables in [celery] section:</p>
  
  <pre><code>celery_app_name = airflow.executors.celery_executor
  celeryd_concurrency = 16
  worker_log_server_port = 8795
  broker_url = amqp://guest:guest@localhost:5672//
  celery_result_backend = amqp://guest:guest@localhost:5672//
  flower_host = 0.0.0.0
  flower_port = 5555
  default_queue = default
  </code></pre>
  
  <p>What am I doing wrong here?</p>