private DeclaredZone readDeclaredZone(Environment environment, Optional<AthenzService> athenzService,
                                          Optional<String> testerNodes, Element regionTag) {
        return new DeclaredZone(environment, Optional.of(RegionName.from(XML.getValue(regionTag).trim())),
                                athenzService, testerNodes,
                                readCloudAccounts(regionTag), readHostTTL(regionTag));
    }
Returns the given non-blank attribute of tag or throw
config-model-api/src/main/java/com/yahoo/config/application/api/xml/DeploymentSpecXmlReader.java