Fix targetcli install error on ubuntu focal

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
diff --git a/lib/cinder b/lib/cinder
index c2e55f9..b1e3d0d 100644
--- a/lib/cinder
+++ b/lib/cinder
@@ -419,7 +419,7 @@
     if [[ "$CINDER_ISCSI_HELPER" == "tgtadm" ]]; then
         install_package tgt
     elif [[ "$CINDER_ISCSI_HELPER" == "lioadm" ]]; then
-        if [[ ${DISTRO} == "bionic" ]]; then
+        if is_ubuntu; then
             # TODO(frickler): Workaround for https://launchpad.net/bugs/1819819
             sudo mkdir -p /etc/target