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
diff --git a/.zuul.yaml b/.zuul.yaml
index 343921e..fe613e6 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -306,7 +306,7 @@
     parent: cinder-tempest-plugin-lvm-barbican-base
     vars:
       devstack_localrc:
-        CINDER_ISCSI_HELPER: lioadm
+        CINDER_TARGET_HELPER: lioadm
 
 - job:
     name: cinder-tempest-plugin-lvm-lio-barbican-centos-8-stream
@@ -378,7 +378,7 @@
     parent: cinder-tempest-plugin-lvm-barbican-base
     vars:
       devstack_localrc:
-        CINDER_ISCSI_HELPER: tgtadm
+        CINDER_TARGET_HELPER: tgtadm
 
 - job:
     name: cinder-tempest-plugin-cbak-s3