Allow to override label bootstrap col class attribute in templates

[exu]

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

[stloyd]

@pjedrzejewski Looks good for me.

[pjedrzejewski]

Thanks Jacek!