Do not lose default route when editing an interface (bnc#883836).

https://bugzilla.novell.com/show_bug.cgi?id=883836
and in fact you can create whole array here like
```
def new_item_default_options
{
  "NETMASK"                                           => NetHwDetection.result["NETMASK" || "255.255.255.0",
  "STARTMODE"                                      => new_device_startmode,
  "DHCLIENT_SET_DEFAULT_ROUTE" => "no"
}
```
it is more readable then this assigning.