[placement][nova][ptg] Resourceless trait filters
> Then we need two steps:
> 1) handle RPs that only have traits but not resources
> 2) allow having the numbered request groups specify only a trait
> without requesting resources.
Nothing stops me from traiting an inventoryless provider today, right?
Do we actually need to involve numbered request groups? Does a concept
of required_in_tree=$trait_list satisfy the requirements?