add k8s_external plugin to CoreDNS configuration (#4704)

roles/kubernetes-apps/ansible/templates/coredns-config.yml.j2

@@ -30,6 +30,9 @@ data:
         forward . /etc/resolv.conf {
           prefer_udp
         }
+{% endif %}
+{% if enable_coredns_k8s_external %}
+        k8s_external {{ coredns_k8s_external_zone }}
 {% endif %}
         cache 30
         loop

roles/kubespray-defaults/defaults/main.yaml

@@ -89,6 +89,8 @@ kube_dns_servers:
 
 dns_servers: "{{kube_dns_servers[dns_mode]}}"
 
+enable_coredns_k8s_external: false
+coredns_k8s_external_zone: k8s_external.local
 
 # Kubernetes configuration dirs and system namespace.
 # Those are where all the additional config stuff goes