Move zombieReaper to its own package

It is not vdsm specific so it's a prime candidate for extraction.

Change-Id: I58e9c834820d98fb3dcffdf06bfc05eb68ea6827
Signed-off-by: Saggi Mizrahi <smizrahi@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/19253
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>