code: function getAdminDbUrl(mongoConnectionString, formUser, poolSize) {
    var parsedMongoUrl = mongoUrlParser.parse(mongoConnectionString);
    parsedMongoUrl.username = formUser.user;
    parsedMongoUrl.password = formUser.pass;
    //according to this: https://docs.mongodb.com/v2.4/reference/user-privileges/#any-database-roles, this type of user should be created in the `admin` database.
    parsedMongoUrl.database = "admin";
    parsedMongoUrl.options = parsedMongoUrl.options || {};
    parsedMongoUrl.options.poolSize = poolSize || MONGODB_DEFAULT_POOL_SIZE;
    var mongourl = mongoUrlParser.format(parsedMongoUrl);
    return mongourl;
  }
docs: Get the url of the mongodb database that will the given formUser to connect.
  @param {string} mongoConnectionString a mongodb url that should at least contain the mongodb hosts
  @param {object} formUser a mongodb user that should have the "readWriteAnyDatabase" role to access any database. This type of user should exist in the mongo `admin` database.
  @param {string} formUser.user the username
  @param {string} formUser.pass the user password
  @param {int} poolSize the poolSize of the mongodb connection. Default to 20.
  @return {string} the mongodb connection url