public List<SearchCluster> getSearchClusters() {
        return Content.getSearchClusters(configModelRepo());
    }
Adds the descendant (at any depth level), so it can be looked up
on configId in the Map.

@param configId   the id to register with, not necessarily equal to descendant.getConfigId().
@param descendant The configProducer descendant to add
config-model/src/main/java/com/yahoo/vespa/model/VespaModel.java