add is_keystone_enabled

This is missing, and makes logic much harder for the grenade use
case.

Change-Id: Ib6670096d190e69273256b87e6a67e2b46945a0d
diff --git a/lib/keystone b/lib/keystone
index 3703008..547646a 100644
--- a/lib/keystone
+++ b/lib/keystone
@@ -496,6 +496,9 @@
     _cleanup_keystone_apache_wsgi
 }
 
+function is_keystone_enabled {
+    return is_service_enabled key
+}
 
 # Restore xtrace
 $XTRACE