kubernetes/preinstall: dns vars cleanup

- Move validation from facts to verify-settings
- Move set_fact to vars/

roles/kubernetes/preinstall/tasks/0040-verify-settings.yml

@@ -220,6 +220,15 @@
   when: dns_mode is defined
   run_once: true
 
+- name: Stop if /etc/resolv.conf has no configured nameservers
+  assert:
+    that: configured_nameservers | length>0
+    fail_msg: "nameserver should not empty in /etc/resolv.conf"
+  when:
+    - upstream_dns_servers | length == 0
+    - not disable_host_nameservers
+    - dns_mode in ['coredns', 'coredns_dual']
+
 - name: Stop if unknown kube proxy mode
   assert:
     that: kube_proxy_mode in ['iptables', 'ipvs']