In MongoDB 3.2, config servers may be a replica set. Hooray!

Please explicitly prevent users from doing the following:

• Create a cluster whose config servers are banana/a:27017,b:27017,c:27017
• Then do sh.addShard( "banana/a:27017" )

Since addShard occurs via a mongos and the mongos must be aware of the current configuration of the config servers, it seems possible to prevent this.