title: Airflow task is not stopped when execution timeout gets triggered
body: <p>I have my own custom operator extends BaseOperator as follows.
  I tried to kill a task if the task runs for more than 30 minutes. 
  timeout seems to be triggered according to a log but the task still continued. </p>
  
  <p>Am I missing something? I checked the official document but do not know what is wrong.
  <a href="https://airflow.apache.org/code.html#baseoperator" rel="nofollow noreferrer">https://airflow.apache.org/code.html#baseoperator</a></p>
  
  <p>My operator is like as follows.</p>
  
  <pre><code>class MyOperator(BaseOperator):
      @apply_defaults
      def __init__(
        self,
        some_parameters_here,
        *args,
        **kwargs):
        *args,
        **kwargs):
        super(MyOperator, self).__init__(*args, **kwargs)
        # some initialization here
  
      def execute(self, context):
        # some code here
  </code></pre>
  
  <p>My task is like as follows.</p>
  
  <pre><code>t = MyOperator(
       task_id='task',
       dag=scheduled_dag,
       execution_timeout=timedelta(minutes=30)
  </code></pre>
  
  <p>I found this error but the task continued.</p>
  
  <pre><code>[2018-04-12 03:30:28,353] {base_task_runner.py:98} INFO - Subtask: [Stage 6:==================================================(1380 + -160) / 1224][2018-04- 
  12 03:30:28,353] {timeout.py:36} ERROR - Process timed out
  </code></pre>
  
  <p>Ref.
  <a href="https://issues.apache.org/jira/browse/AIRFLOW-2385" rel="nofollow noreferrer">https://issues.apache.org/jira/browse/AIRFLOW-2385</a></p>