Setup CoreDNS w/ etcd backend
[CoreDNS][coredns] was designed from the ground up to provide robust, plugin-based DNS server for use in cloud environments. Namely, it serves as the default primary service discovery mechanism for Kubernetes.
Using CoreDNS allows us to have a lightweight DNS server on RockNSM (11 Mb binary is all that's needed!) to facilitate multi-node service discovery. Alternatively, if another existing DNS service is available, this can be used instead. Aligning with the way the Kubernetes manages service discovery also allows us to build new RockNSM features in parallel with the coming Kubernetes support.