*: add "dns" field to the configuration

*: add "dns" field to the configuration

appc/cni#76 added a "dns" field in the result JSON. But before this
patch, the plugins had no way of knowing which name server to return.

There could be two ways of knowing which name server to return:
1. add it as an extra argument ("CNI_ARGS")
2. add it in the network configuration as a convenience (received via
   stdin)

I chose the second way because it is easier. In the case of rkt, it
means the user could just add the DNS name servers in
/etc/rkt/net.d/mynetwork.conf.