title: Add fail safe box URL in Vagrant file
body: <p>I have added local box link in Vagrant file as follows</p>
  
  <pre><code>config.vm.box = "ubuntu1404"
  config.vm.box_url = "http://localserver/ubuntu_trusty64.box"
  </code></pre>
  
  <p>My local server may not be accessible all time. I need to add another box URL for the same box which should accessible from everywhere as fail safe. How can I add another box url ? </p>