allow the vagrant box to be configurable

[mmerickel]

No description provided.

[bfirsh]

What's the use of this? docker-0.8.0 is the only box that exists.

[mmerickel]

Ah well I had a box that's still using the docker-0.7.2 image and encountered some errors when using the newer scripts, so I used this to stay on 0.7.2 for now. The main issue is that the VAGRANT_BOX_URL param introduced in #57 wasn't really useful without this part.

[bfirsh]

Ah, okay. You should in theory be able to set VAGRANT_BOX_URL to http://static.orchardup.com/binaries/vagrant/vagrant-docker-0.7.2-virtualbox.box?

[mmerickel]

Right, but without this PR you cannot tell docker-osx to boot a different box since the URL is only used when your machine doesn't have a box downloaded named docker-0.8.0 right now.

[bfirsh]

Ah sorry, I understand what you mean. Perhaps we could just rename the box "docker" – once you've downloaded it, it automatically updates the Docker version for you.

[freewil]

👍 I don't have an immediate use for this, but I could see it being useful to be able to use two different versions of Docker.

[mmerickel]

I've rebased this onto master. If you'd like to rename the box to "docker" that's fine but it's orthogonal to this PR which allows someone to use any box if necessary.