Replace use of deprecated devstack option

The CINDER_ISCSI_HELPER devstack configuration option was deprecated
and replaced with CINDER_TARGET_HELPER during the Antelope cycle by
change I6578cdc27489b34916cdeb72ba3fdf06ea9d4ad8, so use the
preferred option name here.

Change-Id: Ie32bd7b03ddb94a4b33661205e31954ac5fe145a
1 file changed