At startup, network manager creates a default_connection.nmconnection file. Currently, this means that the user would be unable to use an nmstate config that generated a file of the same name because:
1. we don't set the overwrite option on network configuration files
2. as a workaround for bug 2032717, we delete that file before calling coreos-installer
What we should do instead is to tell IPA whether the user has supplied any custom network config or not, and only pass the --copy-network flag to coreos-installer if there is config to copy. Then we won't need to delete default_connection.nmconnection and we can allow the user to overwrite it.
I don't know to be honest. It was more of a theoretical limitation - a better design approach was suggested and we didn't want to forget about it so we created this bug. I'm most interested in testing for regressions (i.e. can we still specify a network config at all?).