ShutdownInstanceDisks: accept offline secondaries

For secondary node that is offline, we should not consider that the
disk shutdown has failed, as it can never succeed under this cluster
state and (by virtue of the fact that the secondary node is offline)
the disks are already "shutdown".

The patch also fixes a tiny typo.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>