vyatta-cfg-system: rename allow-dhcp-nameservers and change to typeless

Rename allow-dhcp-nameservers to disable-dhcp-nameservers and update
the logic to take the new meaning into account.  The option is now also
typeless, so the node is either absent (default) or present (enabled).

Format 'set / delete system disable-dhcp-nameservers'

Linked to Bug #182 and Bug #308

Bug #314 http://bugzilla.vyos.net/show_bug.cgi?id=314