Fix `useDefaultGroups` option in Groups module #4280

The docs (http://visjs.org/docs/network/groups.html#) indicate that if `useDefaultGroups` is passed as false, it will iterate through the provided groups to use as defaults instead. But the code doesn't seem to match that description, as `useDefaultGroups` is ignored, and the code that handles it being false doesn't seem quite right (it should fetch the full group, not just the color)