Free disk space reporting follows exclusive storage rules

When exclusive_storage is set, nodes report only the space that could be
used to create new disk according to the rules set in the
design-partitioned doc (i.e., non-empty disks are ignored and the space
reserved for rounding/overhead is not reported).

Signed-off-by: Bernardo Dal Seno <bdalseno@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>