Add note about display_name in _populate_instance_names

The None check on instance.display_name in _populate_instance_names
is confusing since it's a required field in the REST API. The check
is really only there now for unit test simplicity, so add a note
to avoid confusion.

Change-Id: I5229892a600ce1526b6842da6082b105bb98e2a0

nova/compute/api.py

@@ -1356,6 +1356,8 @@ class API(base.Base):
         else:
             hostname = None
 
+        # NOTE(mriedem): This is only here for test simplicity since a server
+        # name is required in the REST API.
         if display_name is None:
             display_name = self._default_display_name(instance.uuid)
             instance.display_name = display_name