Allow disk/network HW requirements to speak about the whole set of devices

[happz]

The current assumption is the index matters:

disk:
  - size: "> 20 GB"

The first disk shall have this size. But what if it's "any disk", "all disks" or "no disk"? I think we need at least one new keyword, all:

disk:
  all:
    size: ...
    model-name: ...

• instead of a sequence, disk (or network) could contain the keyword all,
• its content would be effectively the same as one item under disk (or network),
• HW requirements would consider the requirements to apply to all devices of said category.