Changes to certification preseed file to support new multiple-iperf-target feature of Checkbox "network" script. With this change, iperf targets' IP addresses should go in /etc/maas-cert-server/iperf.conf on the MAAS server, in a comma-delimited list on a single line. That line is then passed to the SUT for embedding in /etc/xdg/canonical-certification.conf. If the /e/m-c-s/iperf.conf file is missing, then the SUT uses the gateway address, just as it did before. This must be documented in the documentation project.