Fix scheduler compatibility with Python 3.7

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.

Change-Id: I4e93b0e85500455de415188cc679961035958bd1

heat/engine/scheduler.py

@@ -282,7 +282,10 @@ def wrappertask(task):
     def wrapper(*args, **kwargs):
         parent = task(*args, **kwargs)
 
-        subtask = next(parent)
+        try:
+            subtask = next(parent)
+        except StopIteration:
+            return
 
         while True:
             try:
@@ -317,7 +320,10 @@ def wrappertask(task):
             except:  # noqa
                 subtask = parent.throw(*sys.exc_info())
             else:
-                subtask = next(parent)
+                try:
+                    subtask = next(parent)
+                except StopIteration:
+                    return
 
     return wrapper