Client and RS client ignore _connect=False with auth

MongoClient("mongodb://doesntexist.org") properly raises ConnectionFailure, and MongoClient("mongodb://doesntexist.org", _connect=False) properly does not.

But MongoClient("mongodb://jesse:password@somedomainthatdoesntexist.org", _connect=False) raises ConnectionFailure. Let's fix that, and add a _connect option to MongoReplicaSetClient too.