Add iptables rules for multiple IOCs on single host

The scripts detailed at https://epics-controls.org/resources-and-support/documents/howto-documents/channel-access-reach-multiple-soft-iocs-linux/ use NetworkManager for CentOS, which is disabled in the machines we create. The script was adapted to run it as /sbin/ifup-local. The rule is only created if it doesn't exist. The rule can't be deleted in ifdown-local because the interface is already down and we can't get the IP address that was configured.