Cluster verify: check for nodes/instances with no group

Previously, all nodes and instances would *always* be visited/verified. By
driving the verification by node group now, we will miss nodes and
instances that can't be reached from existing node groups, should that rare
and bogus circumstance ever occur.

We safeguard against that by checking for unreachable nodes and instances
explicitly. (These will not be further verified.)

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