If5c860d1e69aaef9a9236303c370479a7714ad43 attempted to move this default
to lioadm while pinning certain Bionic based jobs to tgtadm.
Unfortunately it missed the legacy dsvm based jobs within various
projects that do not inherit from the devstack-platform-bionic base job
and that are also not covered by devstack's gate.
This change simply forces CINDER_ISCSI_HELPER to tgtadm on Bionic based
hosts to ensure it is always used.
Closes-Bug: #1926411
Change-Id: Ib4b38b45f25575c92fb09b8e97fa1b24af0cc06a