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
Merge "Returns thin pool free space calculated from actual usage"
Merge remote-tracking branch 'origin/master' into jiocloud_devmaster