title: airflow.exceptions.AirflowException: Use keyword arguments when initializing operators
body: <p>I am using Airflow version of 1.9.2 with Python 2.7 in Ubuntu. I tried to inherit from <code>ParentOperator</code> class which works fine itself and to create a class called <code>ChildOperator</code>. But when I create a <code>ChildOperator</code> instance, I think some keyword arguments are missing or messed up here and I am getting this error: </p>
  
  <blockquote>
    <p>airflow.exceptions.AirflowException: Use keyword arguments when
    initializing operators</p>
  </blockquote>
  
  <p>Here is a simplified example:</p>
  
  <pre><code>class ParentOperator(BaseOperator, SkipMixin):
  
      @apply_defaults
      def __init__(self,
               conn_id,
               object,
               args={},
               s3_conn_id=None,
               s3_key=None,
               s3_bucket=None,
               fields=None,
               *args,
               **kwargs
               ):
  
          super(ParentOperator, self).__init__(*args, **kwargs)
          ...
  
  
  class ChildOperator(ParentOperator):
  
      @apply_defaults
      def __init__(self,
               conn_id,
               object,
               args={},
               s3_conn_id=None,
               s3_key=None,
               s3_bucket=None,
               fields=None,
               *args,
               **kwargs
               ):
  
          args=...
          super(ChildOperator, self).__init__(
              conn_id,
              object,
              args=args,
              s3_conn_id=s3_conn_id,
              s3_key=s3_key,
              s3_bucket=s3_bucket,
              fields=fields,
              *args,
              **kwargs
          )
  
  ...
  
  myobjc = ChildOperator(
      conn_id="my_default",
      object=table,
      args={},
      s3_conn_id='s3_postgres_dump',
      s3_key=s3_key,
      s3_bucket=s3_bucket,
      dag=dag,
      task_id="task1"
  )
  </code></pre>
  
  <p>Any idea what is causing this error? Is this more of a Python specific issue? </p>