Nexenta iSCSI driver: fixed volume_migration

Fixed volume_migration updating volume provider_location.
Fixed temporal snapshot deleting after volume migrated.

Change-Id: Id23749cb3724e1510865425232b38b9fc32690f8
Closes-Bug: #1263258
Merge "Nexenta iSCSI driver: fixed volume_migration"
(Test commit) Change README to reflect Jio's fork of Cinder

Change-Id: I3f26c0a736e63725aec24ca2fa12c707b8e6460f