vdsmd.service: require either ntpd or chronyd

Fedora 18 ships with chronyd by default, which conflicts with ntpd. We
do not really care which one of the two is running, as long as the host
clock is synchronized. That's what requiring time-sync.target means.

Change-Id: Ie0605bea6d34c214aea8814a72a03e9ad2883fdb
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/11291
Reviewed-by: Zhou Zheng Sheng <zhshzhou@linux.vnet.ibm.com>
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>