title: Subdag not executed and not shown as 'success'
body: <p>I have been experimenting with 'example_subdag_operator' in airflow. After I changed the 'start_date' to <code>datetime.now()</code> and then manually trigger a dag run. The operator is green, but when I zoomed into the subdag itself, nothing is executed (white in graph view) and 'Run' drop down is empty. \</p>
  
  <p>I use the newest version built from master. I also tested on pip released version. Still the same issue \</p>
  
  <p>Is this a bug? Or I missed something?</p>
  
  <pre><code>from datetime import datetime
  
  from airflow.models import DAG
  from airflow.operators.dummy_operator import DummyOperator
  from airflow.operators.subdag_operator import SubDagOperator
  
  from airflow.example_dags.subdags.subdag import subdag
  
  
  DAG_NAME = 'example_subdag_operator'
  
  args = {
      'owner': 'airflow',
      'start_date': datetime.now(),
  }
  
  dag = DAG(
      dag_id=DAG_NAME,
      default_args=args,
      schedule_interval="@once",
  )
  
  start = DummyOperator(
      task_id='start',
      default_args=args,
      dag=dag,
  )
  
  section_1 = SubDagOperator(
      task_id='section-1',
      subdag=subdag(DAG_NAME, 'section-1', args),
      default_args=args,
      dag=dag,
  )
  
  some_other_task = DummyOperator(
      task_id='some-other-task',
      default_args=args,
      dag=dag,
  )
  
  section_2 = SubDagOperator(
      task_id='section-2',
      subdag=subdag(DAG_NAME, 'section-2', args),
      default_args=args,
      dag=dag,
  )
  
  end = DummyOperator(
      task_id='end',
      default_args=args,
      dag=dag,
  )
  
  start.set_downstream(section_1)
  section_1.set_downstream(some_other_task)
  some_other_task.set_downstream(section_2)
  section_2.set_downstream(end)
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/6ofut.png" rel="nofollow noreferrer">subdag operator success</a></p>
  
  <p><a href="https://i.stack.imgur.com/e0MAM.png" rel="nofollow noreferrer">subdag not executed</a></p>