spec: use the fedora systemd macros

Add the support for the fedora systemd macros introduced in fedora 18.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=850412
Change-Id: I76857a6b9ae1a945511f359333735b9497c5e2b0
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/12285
Reviewed-by: Dan Kenigsberg <danken@redhat.com>