rke2-coredns: inject clusterDomain only if missing

[khushalchandak17]

Summary:

Fix duplication of cluster.local in the Corefile when rendering parameters for the kubernetes plugin.

Before:

Corefile: |-
  .:53 {
      errors
      health {
          lameduck 10s
      }
      ready
      kubernetes  cluster.local  cluster.local in-addr.arpa ip6.arpa {
          pods insecure
          fallthrough in-addr.arpa ip6.arpa
          ttl 30
      }

After:

Corefile: |-
  .:53 {
    errors
    health {
        lameduck 10s
    }
    ready
    kubernetes cluster.local in-addr.arpa ip6.arpa {
        pods insecure
        fallthrough in-addr.arpa ip6.arpa
        ttl 30
    }

[thomasferrandiz]

Hi,
can you include a description of what the PR does?