watch_leave_group not triggering callback on expired members

memcache does not clear group members when members expire.
run_watchers checks group value specifically rather than calling
get_members function which filters dead members. this patch makes
run_watchers call get_members.

Change-Id: Ic96116c40642e98ca4edff2bfdb217578e6cfb45
Closes-Bug: #1392800
Merge "watch_leave_group not triggering callback on expired members"