title: PythonOperator with python_callable set gets executed constantly
body: <pre><code>import airflow
  from airflow import DAG
  from airflow.operators.python_operator import PythonOperator
  from datetime import datetime, timedelta
  from workflow.task import some_task
  
  default_args = {
      'owner': 'airflow',
      'depends_on_past': False,
      'email': ['jimin.park1@aig.com'],
      'email_on_failure': False,
      'email_on_retry': False,
      'retries': 0,
      'retry_delay': timedelta(minutes=1),
      'start_date': airflow.utils.dates.days_ago(0)
      # 'queue': 'bash_queue',
      # 'pool': 'backfill',
      # 'priority_weight': 10,
      # 'end_date': datetime(2016, 1, 1),
  }
  
  dag = DAG('JiminTest', default_args=default_args, schedule_interval='*/1 * * * *', catchup=False)
  
  t1 = PythonOperator(
      task_id='Task1',
      provide_context=True,
      python_callable=some_task,
      dag=dag
  )
  </code></pre>
  
  <p>The actual some_task itself simply appends timestamp to some file. As you can see in the dag config file, the task itself is configured to run every 1 min.</p>
  
  <pre><code>def some_task(ds, **kwargs):
      current_time = datetime.datetime.now().strftime("%Y-%m-%d %H:%M:%S")
      with open("test.txt", "a") as myfile:
          myfile.write(current_time + '\n')
  </code></pre>
  
  <p>I simply tail -f the output file and started up the webserver without the scheduler running. This function was being called and things were being appended to the file when webserver starts up. When I start up the scheduler, on each execution loop, the file gets appended.</p>
  
  <p>What I want is for the function to be executed on every minute as intended, not every execution loop.</p>