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.)


Hmm.. when I run `paver test_bokchoy` locally with nothing passed in (just fasttest), I get the following output:

```
edxapp@precise64:~/edx-platform$ paver test_bokchoy --fasttest
---> pavelib.bok_choy.test_bokchoy
---> pavelib.prereqs.install_prereqs
---> pavelib.prereqs.install_ruby_prereqs
Ruby prereqs unchanged, skipping...
---> pavelib.prereqs.install_node_prereqs
Node prereqs unchanged, skipping...
---> pavelib.prereqs.install_python_prereqs
Python prereqs unchanged, skipping...
No modulestore specified, running tests for split.

Setting up for bok-choy
---> pavelib.utils.test.utils.clean_reports_dir
find /edx/app/edxapp/edx-platform/reports -type f -delete
Checking for mongo, memchache, and mysql...
mongo test --eval 'db.dropDatabase()' > /dev/null
DEFAULT_STORE=split ./manage.py lms --settings bok_choy loaddata --traceback common/test/db_fixtures/*.json
Installed 1 object(s) from 1 fixture(s)
Starting test servers...
DEFAULT_STORE=split coverage run --rcfile=/edx/app/edxapp/edx-platform/common/test/acceptance/.coveragerc -m manage lms --settings bok_choy runserver 0.0.0.0:8003 --traceback --noreload /edx/app/edxapp/edx-platform/test_root/log/bok_choy_lms.log
DEFAULT_STORE=split coverage run --rcfile=/edx/app/edxapp/edx-platform/common/test/acceptance/.coveragerc -m manage cms --settings bok_choy runserver 0.0.0.0:8031 --traceback --noreload /edx/app/edxapp/edx-platform/test_root/log/bok_choy_studio.log
python -m stubs.start youtube 9080  /edx/app/edxapp/edx-platform/test_root/log/bok_choy_youtube.log
python -m stubs.start xqueue 8040 register_submission_url=http://0.0.0.0:8041/test/register_submission /edx/app/edxapp/edx-platform/test_root/log/bok_choy_xqueue.log
python -m stubs.start video 8777 root_dir=/edx/app/edxapp/edx-platform/test_root/data/video /edx/app/edxapp/edx-platform/test_root/log/bok_choy_video_sources.log
python -m stubs.start comments 4567  /edx/app/edxapp/edx-platform/test_root/log/bok_choy_comments.log
python -m stubs.start ora 8041  /edx/app/edxapp/edx-platform/test_root/log/bok_choy_ora.log
Waiting for servers to start...
Checking server 0.0.0.0 on port 8003
Checking server 0.0.0.0 on port 8031
DEFAULT_STORE=split SCREENSHOT_DIR='/edx/app/edxapp/edx-platform/test_root/log' HAR_DIR='/edx/app/edxapp/edx-platform/test_root/log/hars' SELENIUM_DRIVER_LOG_DIR='/edx/app/edxapp/edx-platform/test_root/log' nosetests /edx/app/edxapp/edx-platform/common/test/acceptance/tests --with-xunit --xunit-file=/edx/app/edxapp/edx-platform/reports/bok_choy/xunit.xml --verbosity=2 
```