title: Airflow - CRITICAL - Cannot import authentication module airflow.contrib.auth.backends.ldap_auth
body: <p>I'm using <code>Airflow 1.9</code> and installed it using "<code>pip install apache-airflow[all]</code>". while trying to use LDAP authentication, I'm seeing the below error. </p>
  
  <pre><code>Aug  6 10:26:07 test-airflow airflow: [2018-08-06 10:26:07,364] {__init__.py:64} CRITICAL - Cannot import authentication module airflow.contrib.auth.backends.ldap_auth. Please correct your authentication backend or disable authentication: cannot import name NoValue
  Aug  6 10:26:07 test-airflow airflow: Traceback (most recent call last):
  Aug  6 10:26:07 test-airflow airflow: File "/bin/airflow", line 27, in &lt;module&gt;
  Aug  6 10:26:07 test-airflow airflow: args.func(args)
  Aug  6 10:26:07 test-airflow airflow: File "/usr/lib/python2.7/site-packages/airflow/bin/cli.py", line 678, in webserver
  Aug  6 10:26:07 test-airflow airflow: app = cached_app(conf)
  Aug  6 10:26:07 test-airflow airflow: File "/usr/lib/python2.7/site-packages/airflow/www/app.py", line 161, in cached_app
  Aug  6 10:26:07 test-airflow airflow: app = create_app(config)
  Aug  6 10:26:07 test-airflow airflow: File "/usr/lib/python2.7/site-packages/airflow/www/app.py", line 44, in create_app
  Aug  6 10:26:07 test-airflow airflow: airflow.load_login()
  Aug  6 10:26:07 test-airflow airflow: File "/usr/lib/python2.7/site-packages/airflow/__init__.py", line 67, in load_login
  Aug  6 10:26:07 test-airflow airflow: raise AirflowException("Failed to import authentication backend")
  Aug  6 10:26:07 test-airflow airflow: airflow.exceptions.AirflowException: Failed to import authentication backend
  Aug  6 10:26:07 test-airflow systemd: airflow-webserver.service: main process exited, code=exited, status=1/FAILURE
  Aug  6 10:26:07 test-airflow systemd: Unit airflow-webserver.service entered failed state.
  Aug  6 10:26:07 test-airflow systemd: airflow-webserver.service failed.
  </code></pre>
  
  <p>Here is my <code>airflow.cfg</code>:</p>
  
  <pre><code>[webserver]
  authenticate = True
  auth_backend = airflow.contrib.auth.backends.ldap_auth
  base_url = http://test-airflow.example.com
  </code></pre>