public Set<ClusterSpec> allClusters() {
        return hostSystem().getHosts().stream()
                                      .map(HostResource::spec)
                                      .filter(spec -> spec.membership().isPresent())
                                      .map(spec -> spec.membership().get().cluster())
                                      .collect(Collectors.toCollection(LinkedHashSet::new));
    }
Returns the spedc of all clusters in this
config-model/src/main/java/com/yahoo/vespa/model/VespaModel.java