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: Icae3ec4d1ee2036fbba7b9eb5c03a1c86014fcc0
Merge "Delete iSCSI devices after volume detached"