public ApplicationContainerCluster getContainerCluster(String clusterId) {
        return (ApplicationContainerCluster) root.getChildren().get(clusterId);
    }
Utility functions for testing the ContainerModelBuilder. Note that XML validation will
not be done when using this class

@author gjoranv
config-model/src/test/java/com/yahoo/vespa/model/container/xml/ContainerModelBuilderTestBase.java