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]: 0ef7af76bc/roles/kubespray-defaults/defaults/main.yaml (L104)
[2]: 1afdb05ea9/inventory/sample/group_vars/k8s_cluster/k8s-cluster.yml (L196)

roles/kubernetes-apps/registry/templates/registry-proxy-ds.yml.j2

@@ -27,7 +27,7 @@ spec:
           imagePullPolicy: {{ k8s_image_pull_policy }}
           env:
             - name: REGISTRY_HOST
-              value: registry.{{ registry_namespace }}.svc.{{ cluster_name }}
+              value: registry.{{ registry_namespace }}.svc.{{ dns_domain }}
             - name: REGISTRY_PORT
               value: "{{ registry_port }}"
           ports: