Add note in comment where upstream taskflow change is

The optional task should in the future not be needed
due to an ongoing taskflow review, so denote that in
the async code for future prosperity.

Change-Id: I96ac9c59c10cd61d48aee609150271c72572be7f

glance/async/utils.py

@@ -55,6 +55,9 @@ class OptionalTask(task.Task):
         # The taskflow team is working on improving this and on something that
         # will provide the ability of defining optional tasks. For now, to lie
         # ourselves we must.
+        #
+        # NOTE(harlowja): The upstream change that is hopefully going to make
+        # this easier/built-in is at: https://review.openstack.org/#/c/271116/
         def wrapper(*args, **kwargs):
             try:
                 return func(*args, **kwargs)