Support alt. mongo hosts/ports for tests

Quick little fix so that I can run tests with non-default mongo settings.

test_settings.py, test_prefix.py test_prefix_version.py and test_version.py still require 
appropriate MONGO_* settings in order to work correctly. Ultimately, I'd suggest changing the test code so that it spins up a temporary in-memory mongod instance, rather than relying on a production server for tests and an appropriate agreement of settings between the two.