Sync oslo threadgroup.py to fix wait & stop methods

Sync oslo threadgroup.py to 25ff65e955b7ef1091722224f7c71f14fc971354 so
we include the fix for bug #1196525, which resolves a problem where the
wait and stop methods would not actually wait for/stop all threads in
the thread group.

Change-Id: Ia49d7096180b2942cf49b1f0dacde3c5e933337c
Merge "Sync oslo threadgroup.py to fix wait & stop methods"