public static List<ContentCluster> getContentClusters(ConfigModelRepo pc) {
       List<ContentCluster> clusters = new ArrayList<>();
        for (Content c : getContent(pc))
            clusters.add(c.getCluster());
        return clusters;
    }
Returns indexing cluster implicitly created by this,
or empty if an explicit cluster is used (or if called before the build phase)
config-model/src/main/java/com/yahoo/vespa/model/content/Content.java