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.)


The problem is that the default_modulestore switch is being handled in a different way than all the other options switches. While this could make some sense I think it could be simpler and easier to maintain... I will put together an alternate proposal and post it up here shortly.