Set CINDER_ISCSI_HELPER explicitly for tgtadm job

Required to allow the default of CINDER_ISCSI_HELPER to change for
Ubuntu based hosts to lioadm without breaking these tgtadm jobs.

Change-Id: Ie9ca822f53abf90052ddfae52852b8b78c910ece
diff --git a/.zuul.yaml b/.zuul.yaml
index 27001af..d65e04e 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -119,6 +119,9 @@
       This jobs configures Cinder with LVM, tgt, barbican and
       runs tempest tests and cinderlib tests.
     parent: cinder-tempest-plugin-lvm-barbican-base
+    vars:
+      devstack_localrc:
+        CINDER_ISCSI_HELPER: tgtadm
 
 - job:
     name: cinder-tempest-plugin-basic