vagrant: Simplify provisioning to build from repo

This sets up an idiomatic Go workspace in `/opt/go` with the source shared from the host directory in  `/opt/go/src/github.com/dotcloud/docker` and docker compiled/installed into `/opt/go/bin`.

~~Tested on Virtualbox, but I was unable to test on AWS due to #182.~~

Tested on Virtualbox, AWS, and Rackspace.