Back in the time in which we supported both standalone and replica set for config server, we used to store the config replica set object in "configRS" and the config server nodes connections in "_configServers".
Currently we only support the config server as replica set in ShardingTest and thus the latter is not necessary anymore. Moreover since we are always using replsettest class to instantiate the config server replica set, we can completely delegate the authentication and bridge setting management for the config server to it.

The idea is that shardingtest will only expose the config repl set object "configRS" and a minimal set of helpers function to operate over it.