Add option to use internal Kube-DNS by default

[khaos-one]

Default DNS servers is statically set to Google's 8.8.8.8 and 8.8.4.4, however it seems that one of the main reasons to use Kubernetes VPN is to be able to connect seamlessly with internal K8S resources by their internal names.

Moreover, default Kube-DNS setup on all major clouds properly downstream "external DNS" records, so switching to Kube-DNS does not pose any effects when accessing external internet.

There already is, however, functionality to override VPN's DNS config through VPN_DNS_SRV1 and VPN_DNS_SRV2 env variables, but this is neither documented nor supported on chart level.

Proposed change is to enable chart's users to specify if they want to use internal Kube-DNS by default.