Replace cluster_name by dns_domain (#7923)

`cluster_name` defaults to `dns_domain` value (see [here][1] and [here][2])
but they could have different values.

`dns_domain` should be used here instead of `cluster_name` because the DNS
resolution is configured to use `dns_domain`.

[1]: https://github.com/kubernetes-sigs/kubespray/blob/0ef7af76bc234efa06183e7329e557df87f9c0ee/roles/kubespray-defaults/defaults/main.yaml#L104
[2]: https://github.com/kubernetes-sigs/kubespray/blob/1afdb05ea9e1678b97e0224191d0a9341ce84f41/inventory/sample/group_vars/k8s_cluster/k8s-cluster.yml#L196