title: Airflow : pass parameter from python function to MySQL Operator
body: <p>What I want to do :
  <br>- compare missing ID from MySQL and BigQuery
  <br>- get all data from MySQL where id in missing ID</p>
  
  <pre class="lang-py prettyprint-override"><code>table = 'orders'
  
  def get_missing_ids():
      aws_id = get_aws_id(table)
      bq_id = get_bq_id(table)
      missing_id = [np.setdiff1d(aws_id,bq_id)]
      missing_ids = ', '.join(map(str,missing_id))
      return missing_ids
  
  missing_ids = get_missing_ids()
  
  get_missing_data = MysqlToGCS(
      task_id = 'get_orders_from_aws',
      sql = """select *
          from orders 
          where id in ({{params.missing_ids}})""",
      params = {'missing_ids':missing_ids},
      bucket = 'airflow_bucket',
      filename = 'data/orders/db-orders{{ds}}{}',
      mysql_conn_id = 'aws_readreplica',
      approx_max_file_size_bytes = 100000000,
      google_cloud_storage_conn_id = 'google_cloud_storage_default',
      dag=dag)
  
  def print_done():
      print("done boiiiii")
      time.sleep(60)
  
  task = PythonOperator(
          task_id='done',
          python_callable=print_done,
          dag=dag)
  
  task.set_upstream(get_missing_data)
  </code></pre>
  
  <p>I read about Xcom, but I don't understand how to implement it here.</p>