function loadCachedOverlay(log, callback) {
    return metadata.getObjectMD(managementDatabaseName,
        latestOverlayVersionKey, {}, log, (err, version) => {
            if (err) {
                if (err.is.NoSuchKey) {
                    return process.nextTick(callback, null, {});
                }
                return callback(err);
            }
            return metadata.getObjectMD(managementDatabaseName,
                `configuration/overlay/${version}`, {}, log, (err, conf) => {
                    if (err) {
                        if (err.is.NoSuchKey) {
                            return process.nextTick(callback, null, {});
                        }
                        return callback(err);
                    }
                    return callback(null, conf);
                });
        });
}
Loads the latest cached configuration overlay from the management
database, without contacting the Orbit API.

@param  {werelogs~Logger} log Request-scoped logger
@param  {function} callback Function called with (error, cachedOverlay)

@returns {undefined}