A modified backport of 8a6ee4808782820cc0938519eb3db63a5eb9a2ff

Delete iSCSI devices after volume detached

Previously, after detaching from a volume, the iSCSI device
remains attached on the compute node until all LUNs for a given
IQN are detached -- causing issues when LUNs are reused for
different volumes. This change will delete the device(s)
associated with the detached volume so LUNs can be reused.

Fixes bug 1112483

Change-Id: I76fa257333de9a6d16c7d73708ceb793be1a3b0c