Allow to override label bootstrap col class attribute in templates

Now it's not possible to override e.g. in `src/Sylius/Bundle/WebBundle/Resources/views/Frontend/Cart/summary.html.twig` 

line with label class attribute
```
{{ form_row(form.promotionCoupon, {'label_attr': {'class': 'col-lg-12'}}) }}
```
is always overriden by default value `col-lg-2`