title: How to use mssql_conn_id in Airflow MsSqlOperator?
body: <p>I'm trying to use the MsSqlOperator in my Airflow workflow, but I can't work out how to set the connection string.</p>
  
  <p>I've tried setting mssql_conn_id to be the connection string itself</p>
  
  <pre><code>t2 = MsSqlOperator(
    task_id='sql-op',
    mssql_conn_id='sa:password@172.17.0.2',
    sql='use results; insert into airflow value("airflow","out")',
    dag=dag)
  </code></pre>
  
  <p>I get the error</p>
  
  <pre><code>airflow.exceptions.AirflowException: The conn_id `sa:password@172.17.0.2` isn't defined
  </code></pre>
  
  <p>so I suppose mssql_conn_id needs to be defined. Somewhere. Any ideas?</p>
  
  <p>I'm able to connect to the MS SQL database using sqlalchemy like this:</p>
  
  <pre><code>params = urllib.quote_plus("DRIVER={ODBC Driver 13 for SQL Server};SERVER=172.17.0.2;UID=SA;PWD=password")
  engine = create_engine("mssql+pyodbc:///?odbc_connect=%s" % params)
  
  conn = engine.connect()
  </code></pre>
  
  <p>so I know the server is up and running.</p>