title: Change id of FormBuilder - Symfony
body: <p>I created a registration form using the command formbuilder.
  I tried this but it does not change my identifier</p>
  
  <pre><code>{{form_widget (form.nombreUsuario, {'attr': {'id': 'signup-firstname'}})}}
  </code></pre>
  
  <p>And appears in html:</p>
  
  <pre><code>&lt;input id="fos_user_registration_form_nombreUsuario" type="text" tabindex="1" autofocus="autofocus" placeholder="form.nombreUsuario" required="required" name="fos_user_registration_form[nombreUsuario]"&gt;
  </code></pre>
  
  <p>My RegistrationFormType is:</p>
  
  <pre><code>    namespace Train\UserBundle\Form\Type;
  
  use Symfony\Component\Form\FormBuilderInterface;
  use FOS\UserBundle\Form\Type\RegistrationFormType as BaseType;
  
  class RegistrationFormType extends BaseType
  {
  
      public function buildForm(FormBuilderInterface $builder, array $options)
      {  
          //parent::buildForm($builder, $options);
           $builder-&gt;add('nombreUsuario', 'text', array('required' =&gt; 'true',
                      'attr' =&gt; array(
                          'placeholder' =&gt; 'form.nombreUsuario', 'autofocus' =&gt; 'autofocus', 'tabindex' =&gt; '1'
              )));         
      }
  
      public function getName()
      {
          return 'train_user_registration';
      }
  
  }
  </code></pre>
  
  <p>And my app/config.yml</p>
  
  <pre><code>fos_user:
      db_driver: orm 
      firewall_name: main
      user_class: Train\UserBundle\Entity\User
      registration:
          form:
              type: train_user_registration
  </code></pre>
  
  <p>My UserBundle/services.yml</p>
  
  <pre><code>services:
      train_user.registration.form.type:
          class: Train\UserBundle\Form\Type\RegistrationFormType
          arguments: [%fos_user.model.user.class%]
          tags:
          - { name: form.type, alias: train_user_registration }
  </code></pre>
  
  <p>Thank you very much for your time,</p>
  
  <p>Alejandro</p>