Qa 610 x509 Changes to tests, docs, and certificates for x509 support

This PR has three components:

1) Replacement cert files used for testing
2) Fixes to the x509 test so that it passes and uses the correct username for x509
3) Updates to MongoClient and MongoReplicaSetClient documentation to note that both the certfile and keyfile need to be explicitly specified, even if they are in the same file.