Normally, we should not place instances on nodes that do
not have enough disks. However, there is one exception: if
we failover an instance from an offline node, that node can
well be secondary of that instance---the fact that it is currently
primary proves it has enough disks space. The reason why we have
to handle that case special is that if a node is offline, we sometimes
cannot determine the amount of disk available, hence the conservative
estimation is 0.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Hrvoje Ribicic <riba@google.com>