@Override
        public void doBuild(Content content, Element xml, ConfigModelContext modelContext) {
            Admin admin = content.adminModel != null ? content.adminModel.getAdmin() : null; // This is null in tests only
            content.cluster = new ContentCluster.Builder(admin).build(content.containers, modelContext, xml);
            buildIndexingClusters(content, modelContext,
                                  (ApplicationConfigProducerRoot)modelContext.getParentProducer());
        }
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