Add a real roles method because of FOSUserBundle

### Reason:
`FOS\UserBundle\Model\User` model is merging user's groups roles in `getRoles()` method:

```php
    /**
     * Returns the user roles
     *
     * @return array The roles
     */
    public function getRoles()
    {
        $roles = $this->roles;

        foreach ($this->getGroups() as $group) {
            $roles = array_merge($roles, $group->getRoles());
        }

        // we need to make sure to have at least one role
        $roles[] = static::ROLE_DEFAULT;

        return array_unique($roles);
    }
```

So when `UserAdmin` calls `getRoles()` method to display admin form (when editing a user), user's groups roles are checked too.

### Solution:
A `getRealRoles()` method has been added to `Sonata\UserBundle\Model\User` model and is now used into `UserAdmin` class.