void setupConfigId(String parentConfigId) {
        if (this instanceof AbstractConfigProducerRoot) {
            configId = "";
        } else {
            configId = parentConfigId + subId;
            addConfigId(configId);
        }
        computeConfigId();
    }
Returns the {@link ApplicationConfigProducerRoot} that is the parent of this sub-tree, or null
if this sub-tree has no Vespa parent.
config-model/src/main/java/com/yahoo/config/model/producer/AnyConfigProducer.java