Simplify gnt-os diagnose output

Currently, we always list the api/variants, even if these are empty.
This patch changes so that we make clear distiction for empty values
("[no variants]" versus "[variants: ]"), and we only list variants and
parameters when the OS API indicates they should be supported.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>