@Override
    public ConfigInstance.Builder getConfig(ConfigInstance.Builder builder, String configId) {
        ConfigProducer cp = id2producer.get(configId);
        if (cp == null) return null;

        cp.cascadeConfig(builder);
        cp.addUserConfig(builder);
        return builder;
    }
Use for testing. Use as parent for the config producer(s) you want to test, to test
only a subtree of the producers.

@author gjoranv
config-model/src/main/java/com/yahoo/config/model/test/MockRoot.java