private void addHosts(ApplicationId key, Collection<String> newHosts) {
        for (String host : newHosts) {
            log.log(Level.FINE, () -> "Adding " + host);
            host2ApplicationId.put(host, key);
        }
    }
A host registry with a mapping between hosts (hostname as a String) and some type T
TODO: Maybe we should have a Host type, but using String for now.

@author Ulf Lilleengen
configserver/src/main/java/com/yahoo/vespa/config/server/host/HostRegistry.java