Assorted changes to add new options

Make no_gateway work (in spite of a broken netcfg - you cannot get through
netcfg successfully without the gateway being set, but we clear it in
the late_command)

Added serial console option to cobbler::node

Tidied up cobbler::node kopt code

Made the DHCP range optional; a sensible default is provided for dnsmasq
so that the statically assigned IPs can be served without a dynamic
range being added

Removed superfluous no_default_route from the preseed (if it's in
the preseed it's too late anyway)