Fixup concurrent modification
Fixup concurrent modification

Copy the joined group set before iterating over it and
discard members (just incase the driver isn't discarding)
after leaving future completion (success or failure).

Closes-Bug: #1647942

Change-Id: I43cdb6aeaff53feb6f4a8f53ab4664272bff856f

openstack/tooz,openstack/tooz