Cluster verify: make NV_NODELIST smaller

To cope with increasing cluster sizes, we now make nodes try to contact all
other nodes in their group, and one node from every other group.

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