Fixed default DNS min replica for single node clusters (#8112)

roles/kubernetes-apps/ansible/defaults/main.yml

@@ -3,7 +3,7 @@
 dns_memory_limit: 170Mi
 dns_cpu_requests: 100m
 dns_memory_requests: 70Mi
-dns_min_replicas: 2
+dns_min_replicas: "{{ [ 2, groups['k8s_cluster'] | length ] | min }}"
 dns_nodes_per_replica: 16
 dns_cores_per_replica: 256
 dns_prevent_single_point_failure: "{{ 'true' if dns_min_replicas|int > 1 else 'false' }}"