Does each of the root metadata roles require at least one keyid, and a threshold >= 1? #251

In the root.json part of the spec, it states that it is required to have a role defined for root, targets, snapshot, timestamp, and optionally mirror. However in the section for keyid, it does not explicitly state that each role needs to have at least one keyid. Should it? Presumably we should, otherwise we would allow for unsigned metadata.

Likewise, should we also require that threshold must be greater than or equal to one?