Returns thin pool free space calculated from actual usage

This change adds a hidden method which calculates the thin
pool free space from the data_percent LVM options and reports it
accordingly in _update_volume_stats()

Change-Id: Id6a69644505ca10811db458ea90ed10f643054b2
Closes-Bug: 1249782
(cherry picked from commit 1679acd53d1f0c330edf583afe8b347a7304499c)
Merge "Returns thin pool free space calculated from actual usage" into stable/havana