iSCSI backend for Cinder volumes

As part of multi-backends, add iSCSI cinder backend.

* Add iscsi new backend.
* Allow to run iSCSI in multi-backend configurations via Hiera.
* Unit tests to validate both iSCSI & multi-backends.
Merge pull request #634 from enovance/cinder-iscsi

iSCSI backend for Cinder volumes