[ResourceBundle] Easy create form

it is not always necessary to do a service.

``` yaml
app_user_create:
    path: /users/new
    methods: [GET, POST]
    defaults:
        _controller: app.controller.user:createAction
        _sylius:
            template: App:Backend/User:create.html.twig
            form: App\Byndle\Form\UserType
```
Maybe use ```class_exists```instead, no ?