title: Symfony override configuration from the app/config.yml
body: <p>I have in my <code>app/config.yml</code> this:</p>
  
  <pre><code># Doctrine Configuration
  doctrine:
      dbal:
          driver:   %database_driver%
          host:     %database_host%
          port:     %database_port%
          dbname:   %database_name%
          user:     %database_user%
          password: %database_password%
  
      orm:
          auto_generate_proxy_classes: %kernel.debug%
          auto_mapping: true
  </code></pre>
  
  <p>Now I create my bundle and I want it to override some of this configuration:</p>
  
  <pre><code>doctrine:
      orm:
          resolve_target_entities:
                Acme\UserBundle\Interfaces\User: Acme\MyBundle\Entity\User
  </code></pre>
  
  <p>I want to add this configuration without changing <code>app/config.yml</code></p>