Parallelize tests

[nchammas]

This is a follow-up to #58, where we found that we need some work done to xdist to make it usable for running Flintrock's acceptance tests in parallel.

Once that work is done, we'll be able to add a few options here and we should be all set.

Depends on pytest-dev/pytest-xdist#18.

[nchammas]

Another approach to parallelizing the acceptance tests -- which currently take upwards of 1 hour -- is to launch all the clusters we need in parallel within a shared fixture or something.

Upside: We can do this ourselves right now; no dependency on pytest-dev/pytest-xdist#18.
Downside: Running one acceptance test will spin up all the test clusters required by all the acceptance tests.