Check name of SubDag class instead of class itself

`airflow.operators.SubDagOperator` and
`airflow.operators.subdag_operator.SubDagOperator` are NOT the
same. Airflow needs to check against both classes to determine
if a task is in fact a SubDagOperator. This is because of Airflow's
import machinery. It is *probably* ok to check both classes with
`isinstance()` but the behavior is surprising and to cover our bases
we check for __class__.__name__ and a `subdag` attr.

closes #1168

airflow/models.py

@@ -2255,11 +2255,13 @@ def subdags(self):
         """
         Returns a list of the subdag objects associated to this DAG
         """
-        # Late import to prevent circular imports
-        from airflow.operators import SubDagOperator
+        # Check SubDag for class but don't check class directly, see
+        # https://github.com/airbnb/airflow/issues/1168
         l = []
         for task in self.tasks:
-            if isinstance(task, SubDagOperator):
+            if (
+                    task.__class__.__name__ == 'SubDagOperator' and
+                    hasattr(task, 'subdag')):
                 l.append(task.subdag)
                 l += task.subdag.subdags
         return l