Currently we use templating to set the NodeZero IP address in a number of different configuration files and scripts.
We should move this configuration to a single file (/etc/assisted/rendezvous-host.env) and reference it only from there, e.g. as a systemd environment file.
We also template values like URLs, because it is easier and safer to do this in golang (e.g. to use an IP address that may be either IPv4 or IPv6 in a URL) than in bash. We may need to include all of these variables in the file.
This will enable us to interactively configure the rendezvousIP in a single place.