Passing css ID to form template

There is method `AbstractResource::getResourceFormOptions` which return form options array.

This way we can customize form html ID: 
```php
    public function getResourceFormOptions()
    {
        return array(
            'attr' => array(
                'id' => 'process-form',
            )
        );
    }
```

Due to this part of template, some part of customization made in `getResourceFormOptions` is lost. 

```
<form action="{{ path('fsi_admin_resource', {element: element.id}) }}" method="post" {{ form_enctype(form) }} class="form-horizontal">
```

I propose to use `form_start` twig function.