title: Silex SecurityServiceProvider throws 'Identifier "security.authentication_providers" is not defined.'
body: <p>I can't figure out how to use <code>SecurityServiceProvider</code> in <code>Silex</code>. My configuration is:</p>
  
  <pre><code>$app['security.firewalls'] = array(
      'admin' =&gt; array(
          'pattern' =&gt; '^/_admin/.+',
          'form' =&gt; array('login_path' =&gt; '/_admin/', 'check_path' =&gt; '/_admin/login_check'),
          'logout' =&gt; array('logout_path' =&gt; '/_admin/logout'),
          'users' =&gt; array(
              'admin' =&gt; array('ROLE_ADMIN', '5FZ2Z8QIkA7UTZ4BYkoC+GsR...'),
          ),
      ),
  );
  $app-&gt;register(new Silex\Provider\SecurityServiceProvider());
  </code></pre>
  
  <p>This just throws:</p>
  
  <pre><code>Fatal error: Uncaught exception 'InvalidArgumentException' with message 'Identifier "security.authentication_providers" is not defined.'
  </code></pre>
  
  <p>According to the documentation in some cases when you want to access Security features outside of the handling of a request you have to call <code>$app-&gt;boot();</code> but this isn't my situation.<br>
  If I call <code>$app-&gt;boot();</code> before <code>$app-&gt;register(...)</code> it doesn't raise any exception but it probably doesn't boot at all because then in generating login form Twig throws:</p>
  
  <pre><code>Unable to generate a URL for the named route "_admin_login_check" as such route does not exist.
  </code></pre>
  
  <p>There's <a href="https://github.com/fabpot/Silex/issues/473" rel="noreferrer">an issue a few months ago</a> with probably the same problem but it's closed so I guess it should be fixed now</p>