ironic-agent postinst fails on systemd with no iptables

Where the base install for the image uses systemd but
the iptables service is not present, the postinstall
script for the ironic-agent element can fail. This is
due to 'systemctl disable iptables.service' returning
an error status when the service isn't present. Check
whether the service is enabled using 'systemctl
list-unit-files'.

Change-Id: I8069b062a27caead4097747d7aeae24a5262e117

elements/ironic-agent/post-install.d/80-ironic-agent

@@ -17,7 +17,9 @@ case "$DIB_INIT_SYSTEM" in
         fi
         ;;
     systemd)
-        systemctl disable iptables.service
+        if [[ $(systemctl --no-pager list-unit-files iptables) =~ 'enabled' ]]; then
+            systemctl disable iptables.service
+        fi
         systemctl enable $(svc-map ironic-python-agent).service
         ;;
     sysv)