title: how do I use the --conf option in airflow
body: <p>I am trying to run a airflow DAG and need to pass some parameters for the tasks.</p>
  
  <p>How do I read the JSON string passed as the --conf parameter in the command line <code>trigger_dag</code> command, in the python DAG file.</p>
  
  <p>ex: <code>airflow trigger_dag 'dag_name' -r 'run_id' --conf '{"key":"value"}'</code></p>