The iscsi_helper option was deprecated in favour of target_helper in
Queens, and will be removed in the Stein release.

This also renames the cinder_iscsi_helper variable to
cinder_target_helper, deprecating but still supporting the former name
until the Train release.

Change-Id: Ie38c09b2dd8598f62b0733c8444eec5f6ce3daac