cinder: Default CINDER_ISCSI_HELPER to lioadm

As outlined in bug #1917750 the use of tgtadm in multinode environments
with multiple c-vol services can cause volumes to use duplicate WWNs.

This has been shown to cause some encrypted volume test failures as
os-brick returns a /dev/disk/by-id path to n-cpu that can point to the
wrong underlying volume when multiple volumes with the same WWN are
connected to a host.

There is also some speculation that the duplicate WWNs are also causing
libvirt to fail to detach volumes from instances but as yet this has not
been proven.

This change aims to avoid all of the above by switching the default of
CINDER_ISCSI_HELPER to lioadm for all deployments instead of just EL and
SUSE based deployments.

The Bionic platform job however is pinned to tgtadm as there issues
installing python3-rtslib-fb.

Closes-Bug: #1917750
Change-Id: If5c860d1e69aaef9a9236303c370479a7714ad43

.zuul.yaml

@@ -590,6 +590,9 @@
     description: Ubuntu Bionic platform test
     nodeset: openstack-single-node-bionic
     voting: false
+    vars:
+      devstack_localrc:
+        CINDER_ISCSI_HELPER: tgtadm
 
 - job:
     name: devstack-async

lib/cinder

@@ -88,15 +88,15 @@ CINDER_ENABLED_BACKENDS=${CINDER_ENABLED_BACKENDS:-lvm:lvmdriver-1}
 CINDER_VOLUME_CLEAR=${CINDER_VOLUME_CLEAR:-${CINDER_VOLUME_CLEAR_DEFAULT:-zero}}
 CINDER_VOLUME_CLEAR=$(echo ${CINDER_VOLUME_CLEAR} | tr '[:upper:]' '[:lower:]')
 
-# Centos7 and OpenSUSE switched to using LIO and that's all that's supported,
-# although the tgt bits are in EPEL and OpenSUSE we don't want that for CI
+# Default to lioadm
+CINDER_ISCSI_HELPER=${CINDER_ISCSI_HELPER:-lioadm}
+
+# EL and SUSE should only use lioadm, we continue to allow Ubuntu based
+# deployments to use tgtadm for specific jobs in the cinder-tempest-plugin
 if is_fedora || is_suse; then
-    CINDER_ISCSI_HELPER=${CINDER_ISCSI_HELPER:-lioadm}
     if [[ ${CINDER_ISCSI_HELPER} != "lioadm" ]]; then
         die "lioadm is the only valid Cinder target_helper config on this platform"
     fi
-else
-    CINDER_ISCSI_HELPER=${CINDER_ISCSI_HELPER:-tgtadm}
 fi
 
 # For backward compatibility