title: AirflowException: The conn_id `http_default` isn't defined error trying to run an HTTP operator in Airflow
body: <p>I have an Airflow DAG that, as the last task, needs to send an HTTP request to trigger a Google Cloud Function. I am testing it out but the HTTP task fails.</p>
  
  <p>Task definition:</p>
  
  <pre><code>send_standby_signal = ho.SimpleHttpOperator(
          task_id = 'send-k8s-cluster-standby-signal',
          endpoint = 'https://path-to-function-endpoint', 
          method = 'POST',
          data = '',
          trigger_rule = trigger_rule.TriggerRule.ALL_DONE
  )
  </code></pre>
  
  <p>Error:</p>
  
  <pre><code>[2019-10-02 13:57:29,088] {http_operator.py:87} INFO - Calling HTTP method
  [2019-10-02 13:57:29,122] {__init__.py:1580} ERROR - The conn_id `http_default` isn't defined
  Traceback (most recent call last):
    File "/usr/local/lib/python3.7/site-packages/airflow/models/__init__.py", line 1441, in _run_raw_task
      result = task_copy.execute(context=context)
    File "/usr/local/lib/python3.7/site-packages/airflow/operators/http_operator.py", line 92, in execute
      self.extra_options)
    File "/usr/local/lib/python3.7/site-packages/airflow/hooks/http_hook.py", line 101, in run
      session = self.get_conn(headers)
    File "/usr/local/lib/python3.7/site-packages/airflow/hooks/http_hook.py", line 60, in get_conn
      conn = self.get_connection(self.http_conn_id)
    File "/usr/local/lib/python3.7/site-packages/airflow/hooks/base_hook.py", line 80, in get_connection
      conn = random.choice(cls.get_connections(conn_id))
    File "/usr/local/lib/python3.7/site-packages/airflow/hooks/base_hook.py", line 75, in get_connections
      conns = cls._get_connections_from_db(conn_id)
    File "/usr/local/lib/python3.7/site-packages/airflow/utils/db.py", line 73, in wrapper
      return func(*args, **kwargs)
    File "/usr/local/lib/python3.7/site-packages/airflow/hooks/base_hook.py", line 58, in _get_connections_from_db
      "The conn_id `{0}` isn't defined".format(conn_id))
  airflow.exceptions.AirflowException: The conn_id `http_default` isn't defined
  </code></pre>