private Boolean readUseByDefault(Element spec) {
        String useByDefault = spec.getAttribute("default");
        return (useByDefault.isEmpty()) ?
                null :
                Boolean.parseBoolean(useByDefault);
    }
Builds federation options from a federations options element
@author Tony Vaagenes
config-model/src/main/java/com/yahoo/vespa/model/builder/xml/dom/chains/search/FederationOptionsBuilder.java