probe: Add label to list-chunkservers command

This commit adds printing chunkserver's label to the list-chunkservers
command. Such a label in the "porcelain" is added as the 10th column of
the lizardfs-probe's output. In human-readable mode format of the output
is changed to match the one known from list-disks or list-mounts.

Change-Id: I0ee871ad58c0e5936c5f216f39bca477afbb354e