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