Starting the IPython cluster

@gregcaporaso @jairideout @antgonza This PR is related with https://github.com/biocore/qiime/pull/1616

For some reason, I cannot start an IPython cluster on Jenkins from the `all_tests.py` command, and it is working on my end... I hope this change will, either make Jenkins work, or show some light on what is going on. If this breaks anything, I will revert it ASAP.

Is any of you available to review/merge?

Thanks!