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.