add `signal.is_completed(dag_name)`?

Hi,
  I have a DAG that generates many other DAG's. I would like it to generate at most `N` running DAG's.
Is there a way to check within a `PythonTask` is a DAG is complete or not?
I think something like `signal.is_complete(dag_name)` would be sufficient.

Note, that I don't want to call `signal.join_dags(dag_names)` since I don't know which DAGs will complete first.
thanks!
