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 that were not waited upon 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/28915
Reviewed-by: Dan Kenigsberg <danken@redhat.com>