Handle dns_mode set to 'none' in generate nameservers task (#6825)

When dns_mode was set to 'none' the coredns_server became an empty
string and invalid operation of adding string to list was executed.

roles/kubernetes/preinstall/tasks/0040-set_facts.yml

@@ -131,6 +131,8 @@
         {{ [ skydns_server ] + [ skydns_server_secondary ] }}
       {%- elif dns_mode == 'manual' and not dns_early|bool -%}
         {{ ( manual_dns_server.split(',') | list) }}
+      {%- elif dns_mode == 'none' and not dns_early|bool -%}
+        []
       {%- elif dns_early|bool -%}
         {{ upstream_dns_servers|default([]) }}
       {%- endif -%}
@@ -138,9 +140,9 @@
 - name: generate nameservers to resolvconf
   set_fact:
     nameserverentries:
-      nameserver {{ ( coredns_server + nameservers|d([]) + cloud_resolver|d([])) | unique | join(',nameserver ') }}
+      nameserver {{ ( coredns_server|d([]) + nameservers|d([]) + cloud_resolver|d([])) | unique | join(',nameserver ') }}
     supersede_nameserver:
-      supersede domain-name-servers {{ ( coredns_server + nameservers|d([]) + cloud_resolver|d([])) | unique | join(', ') }};
+      supersede domain-name-servers {{ ( coredns_server|d([]) + nameservers|d([]) + cloud_resolver|d([])) | unique | join(', ') }};
 
 - name: gather os specific variables
   include_vars: "{{ item }}"