Ensure that Vagrant functions in directories relative to root

Certain paths in the Vagrantfile were set relative to the current
working directory. Now they are relative to the Vagrantfile itself. This
allows the `vagrant` command to work from subdirectories within the
project.

Attempts to resolve: #41
Merge pull request #42 from OpenTreeMap/hmc/vagrant-relative

Ensure that Vagrant functions in directories relative to root