Support for cached stack tarball on vagrant

As I pointed out on https://github.com/progrium/dokku/pull/199#issuecomment-25620279, I'm been keeping the stack tarball around while experimenting with Dokku on Vagrant VMs in order to speed things up. With this in place I've reduced a lot the time taken to do the initial bootstaping.

@asm89 [suggested](https://github.com/progrium/dokku/pull/199#issuecomment-25625414) adding it to the Wiki but I thought it makes sense to have it around on the Vagrantfile so that people willing to hack on Dokku can benefit from it as well :) 

What do you guys think?