No targetcli package on Ubuntu Focal, it should use targetcli-fb also
when "$CINDER_ISCSI_HELPER" == "lioadm".
Although Xenial only has targetcli, but Xenial is dropped from CI. And
starting from bionic, Ubuntu uses targetcli-fb to replace targetcli. So
here we can use is_ubuntu to make ubuntu use targetcli-fb.
Change-Id: I6d35b6651d486e716980dcd9f4d693bed560463a