[Story] Attempt to discover DNS name in Kubernetes

[ebaron]

In order to provision wildcard certificates for the agent to use, we need the agent to use a callback URL that includes a hostname reachable from Cryostat instead of just the pod IP. CoreDNS, used by most Kubernetes, defaults to using the IPv4 address with dots replaced by hyphens, but also supports using the pod name as a hostname [1]. We should attempt to discover the hostname using a DNS lookup, provided some hints as config properties.

[1] https://github.com/coredns/coredns/blob/master/plugin/kubernetes/README.md