This allows “gnt-node list” to show the difference between modes marked
offline and nodes with e.g. RPC errors (“(nodata)”). node1 is the
master, node2's node daemon crashed and node3 is marked offline:

$ gnt-node list -o name,offline,dtotal,dfree
Node              Offline    DTotal     DFree
node1.example.com N            1.3T      1.3T
node2.example.com N        (nodata)  (nodata)
node3.example.com Y       (offline) (offline)

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