Add LIO iSCSI backend support using python-rtslib

This patch enables LIO as an iSCSI backend for Cinder, using
python-rtslib.

To enable, set "iscsi_helper = lioadm" in cinder.conf.

This requires python-rtslib 2.1.fb27, which is available from pip.

Implements blueprint lio-iscsi-support
DocImpact

Change-Id: Ifb23de65f26a40997afd6148a1d0be39bcc8d196
Merge "Add LIO iSCSI backend support using python-rtslib"