$::project_config::nodepool_config_file_zuulv3 is set as the fqdn of
the host. So far it is only used for nodepool-launchers.
We wish to have the ability to create custom configuration files for
nodepool-builders too. Add the fqdn variable as the first match for
the builder config. If no more specific config, the default should
remain.
Change-Id: Ibdd8bca3f1e0a1f6709be29c37fe9a696a7bb2d4