DNS forwarder

[remohammadi]

We should also handle dns requests, to be able to resolve node names, which is required by some platforms like kubernetes. Other requests should be forwarded to the given DNS addresses.

If the blacksmith is run in HA mode, the address of the other instances should also be passed to the nodes, and they should respond to dns requests even if they aren't in the leader mode (See #7).

[remohammadi]

SkyDNS is better to be replaced. We had to fork it for our situation. I believe using CoreDNS as a library inside blacksmith shouldn't be so hard, and it should decrease our total maintenance effort.

So, we should listen on 53/udp, reply for <Hostname>s and also <Hostname>.<ClusterName>.