Add __bool__ for classes that implement __nonzero__

__nonzero__ has been renamed to __bool__ in python3.4. So add the
__bool__ magic method that will in turn call the existing __nonzero__
method. This avoids the scheduler going into an infinte loop when
it checks for the no. of tasks remaining.

partial blueprint heat-python34-support

Change-Id: I5de554805a2af9abc15808427ce489720287f6b3

heat/engine/dependencies.py

@@ -71,6 +71,10 @@ class Node(object):
         '''Return True if this node is not a leaf (it requires other nodes).'''
         return bool(self.require)
 
+    def __bool__(self):
+        '''Return True if this node is not a leaf (it requires other nodes).'''
+        return self.__nonzero__()
+
     def stem(self):
         '''Return True if this node is a stem (required by nothing).'''
         return not bool(self.satisfy)

heat/engine/scheduler.py

@@ -283,6 +283,10 @@ class TaskRunner(object):
         """Return True if there are steps remaining."""
         return not self.done()
 
+    def __bool__(self):
+        """Return True if there are steps remaining."""
+        return self.__nonzero__()
+
 
 def wrappertask(task):
     """