loadServers = function(app) {
  app.loadConfigBaseApp(Constants.RESERVED.SERVERS, Constants.FILEPATH.SERVER);
  var servers = app.get(Constants.RESERVED.SERVERS);
  var serverMap = {}, slist, i, l, server;
  for (var serverType in servers) {
    slist = servers[serverType];
    for (i = 0, l = slist.length; i < l; i++) {
      server = slist[i];
      server.serverType = serverType;
      if(server[Constants.RESERVED.CLUSTER_COUNT]) {
        utils.loadCluster(app, server, serverMap);
        continue;
      }
      serverMap[server.id] = server;
      if (server.wsPort) {
        logger.warn('wsPort is deprecated, use clientPort in frontend server instead, server: %j', server);
      }
    }
  }
  app.set(Constants.KEYWORDS.SERVER_MAP, serverMap);
}
Load server info from config/servers.json.