title: Apache airflow sla miss - does not send email
body: <p>I use Apache Airflow and I wanted it to send me an email when there is an sla miss. Here is my configuration <a href="https://i.stack.imgur.com/MHymP.png" rel="nofollow noreferrer">Configuration</a></p>
  
  <p>I created a dag run that misses sla for sure.</p>
  
  <pre><code>from airflow import DAG
  from datetime import datetime, timedelta
  from airflow.operators.bash_operator import BashOperator
  
  args = {
      'owner': 'airflow'
      , 'depends_on_past': False
      , 'start_date': datetime(2018, 8, 17, 0, 0)
      , 'retries': 0
      , 'sla': timedelta(seconds=15)
      , 'email': ['myemail@myemail.com']
      , 'email_on_failure': True
      , 'email_on_retry': True
  }
  
  dag = DAG('sla-email-test'
            , default_args=args
            , max_active_runs=1
            , schedule_interval="@daily")
  
  t1 = BashOperator(
      task_id='timeout',
      bash_command='sleep 60',
      retries=0,
      dag=dag,
  )
  </code></pre>
  
  <p>Unfortunetely it did not send any email
  <a href="https://i.stack.imgur.com/ryWPh.png" rel="nofollow noreferrer">Output</a></p>
  
  <p>What can be a cause, can I use the gui to see logs about it ?</p>