title: Symfony FOSUser Bundle error The child node "db_driver" at path "fos_user" must be configured
body: <p>I have just installed FOSUser bundle and initialized it in <code>AppKernel</code>'s <code>registereBundles()</code> function , when I run </p>
  
  <pre><code>console cache:clear
  </code></pre>
  
  <p>It get</p>
  
  <blockquote>
    <p>[Symfony\Component\Config\Definition\Exception\InvalidConfigurationException]
    The child node "db_driver" at path "fos_user" must be configured.</p>
  </blockquote>
  
  <p>My understanding was that unless or until I am not using a bundle it should not interfere in application flow. I wonder why I get this issue when I have done nothing with FOSUserBundle so far </p>