Currently there is a "--top" option when running swift-recon for
disk usage stats. This option lists the x disks with the highest
disk usage in descending order.
This feature adds a "--lowest" option which does the opposite and
lists the y disks with lowest disk usage in ascending order.
Have also updated the docs section with --top and --lowest options
Change-Id: Ic15d407fe010a31995c2bdd9fb88548a1057f569
66fb207fcb