supervdsm: Add zombiereaper to supervdsm

Supervdsm uses multiprocessing.Process to impersonate other users and
check access. To avoid waiting on the subprocesses, we add all
subprocesses to zombiereaper.

Change-Id: I5333caf7a1162c5d783f556bb9204fb74d1ab469
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=841486
Signed-off-by: Dima Kuznetsov <dkuznets@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/34994
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>