Fix dict.values() PY3 compatible

The dict.values()[0] will raise a TypeError in PY3
as dict.values() doesn't return a list any more in PY3
but a view of list.

TrivialFix.

Change-Id: I01297bb68e45db4d15800d2d42e08560da7346bd

kolla/image/build.py

@@ -846,7 +846,7 @@ class KollaWorker(object):
             return
 
         def list_children(images, ancestry):
-            children = ancestry.values()[0]
+            children = six.next(six.itervalues(ancestry))
             for image in images:
                 if image.status not in [STATUS_MATCHED]:
                     continue