performance optimization in inventory.groups_list()

don't calculate all hosts for every parent group of every group
when that parent group was already in the cache

lib/ansible/inventory/__init__.py

@@ -233,6 +233,7 @@ class Inventory(object):
                 groups[g.name] = [h.name for h in g.get_hosts()]
                 ancestors = g.get_ancestors()
                 for a in ancestors:
+                    if a.name not in groups:
                         groups[a.name] = [h.name for h in a.get_hosts()]
             self._groups_list = groups
         return self._groups_list