Now iscsi target driver assumes that volume name always start with
"volume-". In fact the name can be configured[1]. This change gets
the volume id from volume object directly.
Closes-Bug: #1597317
[1]: 9da9ebb345/cinder/objects/volume.py (L142)
Change-Id: Iaa366fbc4ddc0265255e5a4d2bb9d166a665856c