Chmod kubeconfig to avoid group-readable (#6800)

After upgrading to newer Kubernetes(v1.17 at least), kubectl command
shows the following warning message:

  WARNING: Kubernetes configuration file is group-readable.
  This is insecure. Location: /home/foo/.kube/config

The kubeconfig was copied from {{ artifacts_dir }}/admin.conf with
kubeconfig_localhost feature. It is better to set valid file mode
at getting it on Kubespray.

roles/kubernetes/client/tasks/main.yml

@@ -87,7 +87,7 @@
   copy:
     content: "{{ final_admin_kubeconfig | to_nice_yaml(indent=2) }}"
     dest: "{{ artifacts_dir }}/admin.conf"
-    mode: 0640
+    mode: 0600
   delegate_to: localhost
   connection: local
   become: no