title: How to override configuration of third bundle from a third bundle
body: <p>I'm making a bundle in Symfony 4, but in my bundle I use FOSUserBundle.
  So I would like to defined configuration for FOSUserBundle from my own bundle and not from the main configuration. How to do it ? It doesn't recognize the node</p>
  
  <blockquote>
    <p>There is no extension able to load the configuration for "fos_user" (in C:\wamp64\www\MyProject\src\CMSBundle\DependencyInjection/../Resources/config\fos_user.yaml). Looked for namespace "fos_user", found none</p>
  </blockquote>
  
  <p>And if I load the configuration of FOSUserBundle (FOS\UserBundle\DependencyInjection/Configuration.php) in my CMSExtension.php</p>
  
  <p>I'm getting this message </p>
  
  <blockquote>
    <p>The child node "db_driver" at path "fos_user" must be configured.</p>
  </blockquote>
  
  <p><strong>Structure</strong></p>
  
  <pre><code>/src
      /CMSBundle
          /Controller
          /DependencyInjection
              CMSExtension.php
          /Entity
          /Repository
          /Resources
              /config
                  fos_user.yaml
                  routing.yaml
                  security.yaml
                  services.yaml
          CMSBundle.php
  </code></pre>
  
  <p><strong>src/CMSBundle/DependencyInjection/CMSExtension.php</strong></p>
  
  <pre><code>class CMSExtension extends Extension {
  
      public function load(array $configs, ContainerBuilder $container) {
          $loader = new YamlFileLoader(
              $container,
              new FileLocator(__DIR__ . '/../Resources/config')
          );
          $loader-&gt;load('services.yaml');
          $loader-&gt;load('security.yaml');
          $loader-&gt;load('fos_user.yaml');
      }
  
  }
  </code></pre>
  
  <p><strong>src/CMSBundle/Resourcers/config/fos_user.yaml</strong></p>
  
  <pre><code>fos_user:
      db_driver: orm # other valid values are 'mongodb' and 'couchdb'
      firewall_name: main
      user_class: src\CMSBundle\Entity\User
      from_email:
          address: "%mailer_user%"
          sender_name: "%mailer_user%"
  </code></pre>