This commit reworks the Vagrantfile for Nomad in order to support
straightforward testing on more than one operating system, whilst
retaining the ability to stand up a test cluster running Ubuntu.

The following changes are made:

- Scripts have been extracted from the Vagrantfile into their own shell
  script files, in order that editors lint them.

- All scripts have been edited to lint with no warnings or errors for
  their respective shells.

- Scripts are named according to the operating system and privilege
  level which they run. We prefer to run a whole shell script as root
  versus prefixing (essentially) every command with `sudo` or an
  equivalent.

- The Linux development box has been separated from the test cluster,
  removing some of the more gnarly (and less portable) logic. The Linux
  development box is still primary and autostarts.

- A FreeBSD target has been added. The base box works for both
  Virtualbox and VMWare Fusion.

- A target is added to the GNUmakefile to stand up a test cluster, using
  the default provider, or overriding the provider by setting the PROVIDER
  variable in make:
	- `make testcluster`
	- `make testcluster PROVIDER=vmware_fusion`

- Machines in the test cluster have Avahi configured for zeroconf
  discovery. Each machine can ping each other machine at `hostname.local`
  - for example `nomad-server02.local`, `nomad-client03.local`.