Cluster verify: accept a --node-group option

This will trigger a ClusterVerifyGroup operation only on the specified
group, skipping other groups as well as cluster-wide verifications.

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