title: Passing runtime version using Airflow operator to CMLE
body: <p>I'm using the <a href="https://github.com/apache/airflow/blob/master/airflow/contrib/operators/mlengine_operator.py" rel="nofollow noreferrer">MLEngine operator in contrib</a> (could not find one in none contrib). The problem from the logs is runtime error which is not set properly and then defaults to 1.0.</p>
  
  <blockquote>
    <p>File "/root/.local/lib/python2.7/site-packages/trainer/model.py", line
    210, in train_and_evaluate estimator = tf.estimator.Estimator(model_fn
    = image_classifier, AttributeError: 'module' object has no attribute 'estimator'</p>
  </blockquote>
  
  <p>I think this indicates I should be using TF 1.8 which I am as is indicated both in the trigger:</p>
  
  <pre><code>MLEngineTrainingOperator(
      task_id='ml_engine_training_op1',
      project_id=PROJECT_ID,
      job_id=job_id,
      package_uris=["gs://us-central1-ml-engine-deplo-0bf17ff3-bucket/trainer-0.1.tar.gz"],
      training_python_module=MODULE_NAME,
      training_args=training_args,
      region=REGION,
      scale_tier='BASIC',
      runtime_version = '1.8',
      dag=dag
  )
  </code></pre>
  
  <p>And in the Airflow logs:</p>
  
  <blockquote>
    <p>PendingDeprecationWarning: Invalid arguments were passed to
    MLEngineTrainingOperator. Support for passing such arguments will be
    dropped in Airflow 2.0. Invalid arguments were:
    *args: ()
    **kwargs: {'runtime_version': '1.8'}</p>
  </blockquote>
  
  <p>The version of airflow that is run by Cloud Composer is 1.9.</p>
  
  <p>My question is what's the right way to pass in runtime version to CMLE using the operator?</p>