title: execution_date in airflow: need to access as a variable
body: <p>I am really a newbie in this forum. But I have been playing with airflow, for sometime, for our company.  Sorry if this question sounds really dumb.</p>
  
  <p>I am writing a pipeline using bunch of BashOperators. 
  Basically, for each Task, I want to simply call a REST api using 'curl' </p>
  
  <p>This is what my pipeline looks like(very simplified version):</p>
  
  <pre><code>from airflow import DAG
  from airflow.operators import BashOperator, PythonOperator
  from dateutil import tz
  import datetime
  
  datetime_obj = datetime.datetime
  
  default_args = {
      'owner': 'airflow',
      'depends_on_past': False,
      'start_date': datetime.datetime.combine(datetime_obj.today() - datetime.timedelta(1), datetime_obj.min.time()),
      'email': ['xxxx@xxx.xxx'],
      'email_on_failure': True,
      'email_on_retry': False,
      'retries': 2,
      'retry_delay': datetime.timedelta(minutes=5),
  }
  
  
  current_datetime = datetime_obj.now(tz=tz.tzlocal())
  
  dag = DAG(
      'test_run', default_args=default_args, schedule_interval=datetime.timedelta(minutes=60))
  
  curl_cmd='curl -XPOST "'+hostname+':8000/run?st='+current_datetime +'"'
  
  
  t1 = BashOperator(
      task_id='rest-api-1',
      bash_command=curl_cmd,
      dag=dag)
  </code></pre>
  
  <p>If you notice I am doing <code>current_datetime= datetime_obj.now(tz=tz.tzlocal())</code>
  Instead what I want here is <strong><em>'execution_date'</em></strong> </p>
  
  <p>How do I use <strong>'execution_date'</strong> directly and assign it to a variable in my python file? </p>
  
  <p>I have having this general issue of accessing args. 
  Any help will be genuinely appreciated.</p>
  
  <p>Thanks</p>