title: EasyAdminBundle: error add entity with a composite primary key
body: <p>I have a Vehicule entity which I want to configure it with <strong>easyAdmin Bundle</strong>.
  my entity has two primary keys: <strong>immatriculation and user(from entity users : id_user)</strong>:</p>
  
  <pre><code> //Vehicule Entity
   /**
   * @ORM\Id
   * @ORM\Column(type="string", name="num_immat")
   */
  private $immatriculation;
   /** 
   * @ORM\Id
   * @ORM\ManyToOne(targetEntity="CMN\PortailFretBundle\Entity\User", inversedBy="vehicules")
   * @ORM\JoinColumn(name="id_user", referencedColumnName="id_user")
   */
  private $user;
  </code></pre>
  
  <p>I added my entity to the admin.yml file:</p>
  
  <pre><code>entities:
      Vehicule:
          label: "vehicules"
          class: CMN\PortailFretBundle\Entity\Vehicule
          form:
              fields: ['immatriculation','longueur', 'user']
          edit:
              fields: ['immatriculation','longueur', 'user']
          list:
              fields:
                   - { property: 'immatriculation'}
                   - { property: 'longueur'}
  </code></pre>
  
  <p>When I try to see the Vehicule management I get this error:</p>
  
  <pre><code>Single id is not allowed on composite primary key in entity CMN\PortailFretBundle\Entity\Vehicule
  </code></pre>
  
  <p>Does the easyAdminBundle doesn't support two primary keys ?? How can I do it?</p>