Use testtools, fixtures and testrepository for parallel testing support

We want to start using testrepository as our test runner instead of nose so that we can start running tests in parallel. Initial testing on unittests show an Yx increase in speed where Y is the number of CPUs present on the box.

As part of getting set for that, we need to start using testtools and fixtures so provide the plumbing and test isolation needed for automatic parallelization. The work can be done piecemeal - with testtools and fixtures being added first, and then tox/run_tests being ported to us testr/subunit instead of nose.