title: Broken DAG: [/airflow/dags/a.py] Can't decrypt `extra` params for login=None, FERNET_KEY configuration is missing
body: <p>I started Airflow with no FERNET_KEY. Once I realised it, I did the following:
  <a href="https://airflow.apache.org/configuration.html#connections" rel="noreferrer">https://airflow.apache.org/configuration.html#connections</a></p>
  
  <pre><code>pip install apache-airflow[crypto]
  
  from cryptography.fernet import Fernet
  fernet_key= Fernet.generate_key()
  print(fernet_key)
  </code></pre>
  
  <p>took the key and placed it in <code>airflow.cfg</code> and then called <code>airflow initdb</code>, but the error still appears.</p>
  
  <p>What am I doing wrong?</p>
  
  <p>When I do:</p>
  
  <pre><code>airflow webserver -D
  </code></pre>
  
  <p>I get:</p>
  
  <pre><code>  File "/usr/local/lib/python2.7/dist-packages/airflow/models.py", line 713, in extra_dejson
      if self.extra:
    File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/attributes.py", line 293, in __get__
      return self.descriptor.__get__(instance, owner)
    File "/usr/local/lib/python2.7/dist-packages/airflow/models.py", line 632, in get_extra
      return fernet.decrypt(bytes(self._extra, 'utf-8')).decode()
    File "/usr/lib/python2.7/dist-packages/cryptography/fernet.py", line 101, in decrypt
      raise InvalidToken
  </code></pre>
  
  <p>The log indicates that there is issue with this code:</p>
  
  <pre><code>def get_conn(conn_id, session=None):
      conn = (session.query(Connection)
                     .filter(Connection.conn_id == conn_id)
                     .first())
      return conn
  
  
  def my_python_function():
     conn = get_conn('s3connection')
     key_id = conn.extra_dejson.get('aws_access_key_id')
     secret_key = conn.extra_dejson.get('aws_secret_access_key')
     default_region = conn.extra_dejson.get('region_name')
     return key_id,secret_key,default_region
  </code></pre>