title: can airflow dag run sequential without dependency
body: <p>i would like to create a workflow that starts after past execution have finished but not depends on their success status. meaning dags would be scheduled sequentially, without any dependency on past statuses.</p>
  
  <p>i.e.:
  executions by order:</p>
  
  <p>2017-03-09 15:00:00 success</p>
  
  <p>2017-03-09 16:00:00 failed</p>
  
  <p>2017-03-09 17:00:00 success</p>
  
  <p>2017-03-09 18:00:00 success</p>
  
  <p>how can i do it using Airflow?
  (i want it to be the same for backfill)</p>