[docs] Local installation for testing

Created by: nemesisdesign

I think it would be better to replace the section named "Install OpenWISP2 locally (laptop, desktop pc)" with a section that explains how to install OpenWISP 2 in a virtualbox VM, the same task we have asked GCI students to perform and for which they encountered all the same issues.

We can use their experience to prepare a step by step tutorial for this case that would probably work better than the kind of suggestion we give now.