Add list commands for datacenter and clusters

[burmanm]

To provide a better experience than "kubectl get cassdc", "kubectl get k8ssandracluster", there should be a method to list all the available clusters and datacenters in the target Kubernetes cluster.

This could take something in the form of:

cluster1
  dc1                   <data-plane>            <up/down>
    r1                  <zone>                  <up/down>
    r2                  <zone>                  <up/down>

As well as other info we find relevant. up/down in this case the amount of pods up and down. (or up/total).

With -wide option, perhaps we could even list a status of individual pods also, thus replacing the need for "kubectl get pods".

┆Issue is synchronized with this Jira Task by Unito
┆Issue Number: K8CL-2