volume/iscsi: Fix typo and use real default iscsi_helper in tests

The default value for iscsi_helper isn't correctly configured in
volume::iscsi. This commit fix also a typo in the params class.

Closes-Bug: 1341091
Change-Id: Idcad78c78405c17518b8f4fe75e9954107b386f9
(cherry picked from commit 0e076dec1111c4573db59f524ddb36f5f36566e6)