private boolean hostAlreadyTaken(String host, ApplicationId key) {
        return host2ApplicationId.containsKey(host) && !key.equals(host2ApplicationId.get(host));
    }
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