private Optional<Reader> optionalFile(String file) {
        if (zkApplication.exists(Path.fromString(USERAPP_ZK_SUBPATH).append(file)))
            return Optional.of(getUserAppData(file));
        else
            return Optional.empty();
    }
Returns readers for all the children of a node.
The node is looked up relative to the location of the active application package in zookeeper.
configserver/src/main/java/com/yahoo/vespa/config/server/zookeeper/ZKApplicationPackage.java