title: apt-get update error related with kubeadm
body: <p>I am suffering below problem.</p>
  
  <p>I use ubuntu 16.04. I am following guide document, but it does not works for me.</p>
  
  <pre><code># cat /etc/apt/sources.list.d/kubernetes.list
  deb http://apt.kubernetes.io/ kubernetes-xenial main
  
  # curl -s https://packages.cloud.google.com/apt/doc/apt-key.gpg | apt-key add -
  OK
  
  # apt-get update
  Hit:1 http://kr.archive.ubuntu.com/ubuntu xenial InRelease
  Hit:2 http://kr.archive.ubuntu.com/ubuntu xenial-updates InRelease                            
  Hit:3 http://kr.archive.ubuntu.com/ubuntu xenial-backports InRelease                                  
  Hit:4 http://security.ubuntu.com/ubuntu xenial-security InRelease       
  Get:5 https://packages.cloud.google.com/apt kubernetes-xenial InRelease 
  [8993 B]
  Ign:5 https://packages.cloud.google.com/apt kubernetes-xenial InRelease
  Fetched 8993 B in 1s (5258 B/s)
  Reading package lists... Done
  W: GPG error: https://packages.cloud.google.com/apt kubernetes-xenial 
  InRelease: The following signatures couldn't be verified because the public 
  key is not available: NO_PUBKEY 6A030B21BA07F4FB
  W: The repository 'http://apt.kubernetes.io kubernetes-xenial InRelease' is 
  not signed.
  N: Data from such a repository can't be authenticated and is therefore 
  potentially dangerous to use.
  N: See apt-secure(8) manpage for repository creation and user configuration 
  details.
  </code></pre>
  
  <p>I want to upgrade kubeadm version.
  How to fix this?</p>