Default values for variable dns_servers and dns_domain are set in two files: (#3999)

values from inventory in roles/kubespray-defaults/defaults/main.yml
hardcoded values in roles/container-engine/defaults/main.yml

dns_servers set empty in roles/container-engine/defaults/main.yml and skydns_server not set in docker_dns_servers variables
also set default value for manual_dns_serve

another variables in roles/container-engine/defaults not need to set

roles/container-engine/defaults/main.yml

----
-## DNS
-dns_domain: cluster.local
-dns_servers: []
-upstream_dns_servers: []
-searchdomains: []

roles/kubespray-defaults/defaults/main.yaml

@@ -63,7 +63,7 @@ enable_nodelocaldns: False
 nodelocaldns_ip: 169.254.25.10
 
 # Should be set to a cluster IP if using a custom cluster DNS
-# manual_dns_server: 10.x.x.x
+manual_dns_server: ""
 
 # Can be docker_dns, host_resolvconf or none
 resolvconf_mode: docker_dns