Fix scheduler compatibility with Python 3.7 harder

Always catch StopIteration, rather than allowing it to bubble up a level.
This is neccessary due to PEP0479, wherein it was decided that Python will
start converting this exception to a RuntimeError when it bubbles up,
beginning with Python 3.7.

This is a follow-up to f7edd0ba2d, which
missed one case of where StopIteration could be raised.

Change-Id: I34bc32e7477bcaa0ddfe30179f1008b1be97c0a3

heat/engine/scheduler.py

@@ -318,7 +318,10 @@ def wrappertask(task):
                 parent.close()
                 raise
             except:  # noqa
-                subtask = parent.throw(*sys.exc_info())
+                try:
+                    subtask = parent.throw(*sys.exc_info())
+                except StopIteration:
+                    return
             else:
                 try:
                     subtask = next(parent)