netconf.ifcfg: include CONFFILE_HEADER in route/rule files

Include the
    # Generated by VDSM version x.y.z
header in all vdsm-written ifcfg files.

Change-Id: Ia49754b85797168df056ebd9acf81c151ad3980f
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/24103
Reviewed-by: Assaf Muller <amuller@redhat.com>
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>