public static boolean isStorage(ServiceCluster cluster) {
        return ServiceType.STORAGE.equals(cluster.serviceType());
    }
Note that a search node service cluster (service type searchnode aka proton) is
always accompanied by a storage node service cluster, but not vice versa.

@return true iff the service cluster consists of storage nodes (proton or vds).
orchestrator/src/main/java/com/yahoo/vespa/orchestrator/model/VespaModelUtil.java