Move clear_volume method to volume.utils

clear_volume was almost the same in LVM and BlockDevice drivers.
So it has been moved to utils.

Change-Id: Iadb6b8d01cf500109bb48b0d2c817109918519e0
Merge "Move clear_volume method to volume.utils"
(Test commit) Change README to reflect Jio's fork of Cinder

Change-Id: I3f26c0a736e63725aec24ca2fa12c707b8e6460f