Update paver bok_choy (test) commands to reflect new default vars.

@clytwynec @nasthagiri 

This takes into account recent changes for how bok-choy commands are composed in paver. (Should also bring Jenkins back to green.)


Yea.  Ideally, the logic for setting the default would be [here](https://github.com/edx/edx-platform/blob/master/pavelib/utils/test/suites/bokchoy_suite.py#L31) rather than in the task, as that is where the rest of the defaults are set.