PYTHON-525 Use MongoClient in replica set tests.

Connect the MongoClient to the replica set if one is running,
otherwise connect it to the standalone. Only continue to use
MongoReplicaSetClient when testing the specific behavior of
that deprecated class.

Also suppress some warnings when running tests, and better
determine whether to test IPv6.
Update JSON test files from spec repo.

Tests updated to handle hostname down-casing and the replacement
of "clusterType" with "topologyType".

https://github.com/mongodb/specifications/tree/master/source/server-discovery-and-monitoring/tests
Use lower() to canonicalize hostnames.

Normalize hostname to lowercase, since DNS is case-insensitive:

http://tools.ietf.org/html/rfc4343

This prevents useless rediscovery if "foo.com" is in the seed list but
"FOO.com" is in the ismaster response.