title: Apache Airflow - BigQueryOperator: How to dynamically set destination_dataset_table partition
body: <p>I need a BigQueryOperator task like the following one: in which I need to save result from a query to a partitioned table. However, the <code>"month_start"</code> need to be derived from the actual DAG <code>execution_date</code>. I couldn't find any documents or examples on how to read the execution_date in my DAG definition script (in Python).   Looking forward to some help here. </p>
  
  <p>FYR: I'm with Airflow 1.8.2</p>
  
  <pre><code> t1_invalid_geohash_by_traffic =  BigQueryOperator(
                                          task_id='invalid_geohash_by_traffic',
                                          bql='SQL/dangerous-area/InvalidGeohashByTraffic.sql',
                                          params = params,
                                          destination_dataset_table=
                                          'mydataset.mytable${}'.format(month_start),                                      write_disposition='WRITE_TRUNCATE',
                                          bigquery_conn_id=CONNECTION_ID,
                                          use_legacy_sql=False
                                      )
  </code></pre>