public Collection<String> allHosts() {
        return applications.values().stream()
                .flatMap(app -> app.getModel().getHosts().stream()
                        .map(HostInfo::getHostname))
                .distinct()
                .toList();
    }
Returns the application for the given version, or empty if not found
configserver/src/main/java/com/yahoo/vespa/config/server/application/ApplicationVersions.java