Support DHClient is able to custom prefix DNS instead of using VPN user

[zero88]

Need to check if udhcpc in openwrt is able to do it also

https://github.com/qweio/iot-vpn/blob/6bf7b0b97669958aa786708cfc0cd1c2a7adf2a9/client/python/src/client/cmd_client.py#L391

https://github.com/qweio/iot-vpn/blob/8676d7ffd16e634be4365c655ced012616334a19/client/python/src/client/resources/dhclient-vpn.conf.tmpl#L15-L16