cluster verify and instance disks on offline nodes

Currently, cluster-verify says:

- ERROR: instance instance14: couldn't retrieve status for disk/0 on node3: node offline
- ERROR: instance instance14: instance lives on offline node(s) node3
- ERROR: instance instance15: couldn't retrieve status for disk/0 on node3: node offline
- ERROR: instance instance15: instance lives on offline node(s) node3

This is redundant as the “lives on offline node” message should be all we need to
understand the cluster situation.

The patch fixes this and also corrects a very old idiom.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Stephen Shirley <diamond@google.com>