title: airflow: how to only send email alerts when all retries fail
body: <p>I have the following default args for a airflow dag:</p>
  <pre class="lang-py prettyprint-override"><code>DEFAULT_ARGS = {
      'owner': 'me',
      'depends_on_past': False,
      'email': ['me@me.com'],
      'email_on_failure': True,
      'retries': 4,
      'retry_delay': timedelta(seconds=5)
  }
  </code></pre>
  <p>Each time when a specific job attempt fails, I got an email alert. However, is it possible to ask airflow to only send alerts when all the retries/attempts fail?</p>