Bp cl/add test build role

@jzoldak  @benpatterson 

https://openedx.atlassian.net/browse/TE-433

(The commits can be squashed before merging, I just didn't want to lose the comments before review was done)

I have tested this using the test_role Vagrant file.  The tests do take several minutes to run.
@clytwynec I think test_suite_args and test_shard_args can be removed at this point.