Source Route script now checks that the DHCP response is whole

Previously we assumed that every DHCP response contains a
gateway. Clearly that's wrong :(

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=994600
Change-Id: I3400b1ef736082ed3d5c42edf8ed5d66e94cce98
Signed-off-by: Assaf Muller <amuller@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/17793
Reviewed-by: Dan Kenigsberg <danken@redhat.com>