Remove requirement for variants on OS API v15+

This removes:

- the check in backend that such OSes have a variants file or if it
  exists that is non-empty; in order for this to work, we also rework
  the logic in backend._TryOSFromDisk to allow for optional OS files
- the check in cluster verify such OSes to have a non-empty variant
  list (the check for consistent variants is still kept)

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>