Sprout - appliance spring

Django/celery based appliance spring caretaker.

* Configure templates
* Deploy appliances on-demand
* Keep a pool of spinned appliances
* Spread the appliances across providers

More informations in the MOJO document I posted recently.
This seems backwards. Sprout support seems useful for all runs, not just parallelized runs, so it makes more sense to me for sprout to be something any py.test process can enable, so the slaves probably should be the part using sprout.