title: SecurityServiceProvider - can't logout
body: <p>The login form works fine but the logout option doesn't 
  Here is the code I use </p>
  
  <pre><code>$app-&gt;register(new Silex\Provider\SecurityServiceProvider(), array(
   'security.firewalls' =&gt; array( 
    'admin' =&gt; array(
     'pattern' =&gt; '^/admin',
     'form' =&gt; array( 'login_path' =&gt; '/passport/login', 'check_path' =&gt; '/admin/login_check' ),
     'logout' =&gt; array('logout_path' =&gt; 'admin/logout'),
      'users' =&gt; array(
      // raw password is foo
      'admin' =&gt; array('ROLE_ADMIN', '5FZ2Z8QIkA7UTZ4BYkoC+GsReLf569mSKDsfods6LYQ8t+a8EW9oaircfMpmaLbPBh4FOBiiFyLfuZmTSUwzZg=='),
     ),
    ),
   )));
  $app-&gt;run();
  </code></pre>
  
  <p>Then in one of the templates in the admin I have </p>
  
  <pre><code> &lt;a href="{{path('admin_logout')}}"&gt; Log Out&lt;/a&gt;
  </code></pre>
  
  <p>Which produce this url - /admin/logout</p>
  
  <p>When I click on the link I get <code>Unable to find the controller for path "/admin/logout". The route is wrongly configured</code></p>
  
  <p>What I'm doing wrong?</p>