[master] fallback to default icon_color in collection buttons

> I think in the WidgetCollectionFormTypeExtension the default icon_color, which
> could be defined in the config.yml, is not used as fallback.
> 
> Please insert this patch into branch v2.2.x (and apply to master, too).
> Thanks :-)

See https://github.com/phiamo/MopaBootstrapBundle/issues/472