Support for OpenStack Stein

Update latest release to ship with a version 4 to stein, ensuring
that charm does not try to keep upgrading gnocchi.

Change-Id: Icbd47cd8efaf10403a8bd9b32c35cf5169c4d886

src/lib/charm/openstack/gnocchi.py

@@ -224,7 +224,7 @@ class GnocchiCharm(GnochiCharmBase):
         'gnocchi-common': collections.OrderedDict([
             ('2', 'mitaka'),
             ('3', 'pike'),
-            ('4', 'rocky'),
+            ('4', 'stein'),
         ]),
     }