make sure group_names is always sorted

this makes it consistent with previous ansilbe versions and other
paths that create the group_names variable

lib/ansible/vars/__init__.py

@@ -362,7 +362,7 @@ class VariableManager:
         variables['playbook_dir'] = loader.get_basedir()
 
         if host:
-            variables['group_names'] = [group.name for group in host.get_groups() if group.name != 'all']
+            variables['group_names'] = sorted([group.name for group in host.get_groups() if group.name != 'all'])
 
             if self._inventory is not None:
                 variables['groups']  = dict()