title: can we parameterize the airflow schedule_interval dynamically reading from the variables instead of passing as the cron expression
body: <p><strong>can we parameterize the airflow schedule_interval dynamically reading from the airflow variables instead of passing directly as the cron expression</strong></p>
  <p>I have passed the following way as per airflow documentation
  args = {
  'owner': 'pavan',
  'depends_on_past': False,
  'start_date': datetime(2020, 1, 15),
  'email_on_failure': True,
  'email_on_retry': False,
  'retries': 0,
  'on_failure_callback': notify_email
  }</p>
  <p>with DAG(dag_id=DAG_NAME, default_args=args, schedule_interval='* 1 * * *', catchup=False) as dag:</p>