title: Impossible to call set() on a frozen ParameterBag
body: <p>In my config.yml I have this:</p>
  
  <pre><code>parameters:
      gitek.centro_por_defecto: 1
  </code></pre>
  
  <p>Now, I want to change this value from my controller using a form, like this:</p>
  
  <pre><code>public function seleccionAction(Request $request)
  {
    $entity  = new Centro();
    $form = $this-&gt;createForm(new SeleccionType(), $entity);
    $centro = $this-&gt;container-&gt;getParameter('gitek.centro_por_defecto');
  
    if ($this-&gt;getRequest()-&gt;getMethod() == 'POST') {
      $form-&gt;bind($this-&gt;getRequest());
      if ($form-&gt;isValid()) {
        $miseleccion = $request-&gt;request-&gt;get('selecciontype');
        $this-&gt;container-&gt;setParameter('gitek.centro_por_defecto', $miseleccion['nombre']);
  
        // return $this-&gt;redirect($this-&gt;generateUrl('admin_centro'));
      }
    }
  
    return $this-&gt;render('BackendBundle:Centro:seleccion.html.twig', array(
      'entity' =&gt; $entity,
      'form'   =&gt; $form-&gt;createView(),
      ));
  }
  </code></pre>
  
  <p>I´m getting <code>Impossible to call set() on a frozen ParameterBag.</code> error all the time. 
  Any help or clue?</p>