title: clear an upstream task in airflow within the dag
body: <p>i have a task in an airflow DAG. it has three child tasks. unfortunately, there are cases where this parent task will succeed, but two of the three children will fail (and a retry on the children won't fix them).</p>
  
  <p>it requires the parent to retry (even though it didn't fail).</p>
  
  <p>so i dutifully go into the graph view of the dag run and 'clear' this parent task and all downstream tasks (+recursive).</p>
  
  <p>is there a way i can do this within the dag itself?</p>