Vagrantfile: Debian/Ubuntu + vagrant cachier multi-machine configuration

Update the Vagrantfile so that it is possible (and easier) to spin up Debian
7, Ubuntu 12.04 and Ubuntu 14.04 VMs. Although Vagrant allows spinning up all
of them simultaneously, there may be conflicts assigning host names and IP
addresses.

This Vagrantfile also enables the vagrant-cachier plugin (if installed) to
cache downloaded packages across subsequent runs of the same VM.

Running "vagrant up" will bring up only the Debian VM since it is the
officially supported Linux distributions. Ubuntu 12.04 and Ubuntu 14.04 must
be started explicitly by running either "vagrant up precise" or "vagrant up
trusty".