title: Sonata admin ignores "required" attribute
body: <p>I am working on a Symfony 2.7 application that uses the Sonata admin bundle.  In one of my entities' <code>configureFormFields()</code> method, I have the following:</p>
  
  <pre><code>        -&gt;add('market',
              'entity',
              array(
                  'multiple' =&gt; true,
                  'class' =&gt; 'MyCompany\AppBundle\Entity\Market',
                  'choices' =&gt; $query = $this
                      -&gt;entityManager
                      -&gt;getRepository(Market::class)
                      -&gt;findBy(['status' =&gt; 100])
              ,
                  'placeholder' =&gt; 'no_selection',
                  'required' =&gt; true,
                  'label' =&gt;  $this-&gt;trans('country_of_origin', [], 'messages'),
                  'attr' =&gt; array(
                      'class' =&gt; 'jsb_ jsb_GetDealersForCountry jsb_HideOtherTabs',
                      'data-jsb' =&gt; json_encode(array(
                          'url' =&gt; $this-&gt;getRequest()-&gt;getBaseUrl() . '/dealers/country/id/'
                      )),
                  )
              )
          )
  </code></pre>
  
  <p>... but I find that I am able to save without any value in the 'market' field. </p>
  
  <p>Here is the annotation over the market property in my entity:</p>
  
  <pre><code>/**
   *
   * @ORM\ManyToMany(targetEntity="MyCompany\AppBundle\Entity\Market", cascade={"persist"})
   *
   */
  private $market;
  </code></pre>
  
  <p><strong>So anyway, is there a simple way to make this field truly mandatory?</strong></p>