title: How to avoid Airflow to backfill when using trigger_dag?
body: <p>I want to create a DAG that will only run upon an external trigger (i.e., using the 'airflow trigger_dag ' command). However, when I do this, I see multiple 'scheduled_xxx' DagRuns in addition to the 'manual_xxx' that I want. I am assuming the scheduled_xxx DagRuns are created to backfill?</p>
  
  <p>Is there a way to only have the 'manual_xxx' DagRun created and no 'scheduled_xxx' DagRuns?</p>
  
  <p>I tried different values for start_date (past, datetime.now() and future but got the same result. Here's my toy DAG ...</p>
  
  <pre><code>from airflow import DAG
  from airflow.operators.bash_operator import BashOperator
  from datetime import datetime, timedelta
  
  default_args = {
   'start_date': datetime.now(),   (also tried past and future dates)
   'schedule_interval': None,
   'depends_on_past': False,
  }
  
  dag = DAG('my_test_dag', default_args=default_args)
  
  date_task = BashOperator(
   task_id='date',
   bash_command='date',
   dag=dag)
  </code></pre>
  
  <p>This is how I am issuing the trigger_dag command ...</p>
  
  <pre><code>airflow trigger_dag my_test_dag
  </code></pre>