Allow extra pip packages to be installed on provisioning

TP: https://rockabox.tpondemand.com/entity/6503

Linked to:
https://github.com/rockabox/rbx_web/pull/2016
https://github.com/rockabox/Rockabox.wsgi/pull/2
https://github.com/rockabox/fab-tasks/pull/42
https://github.com/rockabox/rbx/pull/153
defaults?