Remove the NOMAD_TEST_RKT flag as a guard for rkt tests. Still require
Linux, root, and rkt to be installed. Only check for rkt installation
once in hopes of speeding up rkt tests a bit.

`make release` and `make prerelease` set a `release` tag to disable
enabling the `mock_driver`

Broke due to a change in go-bindata-assetfs

We make a HashiCorp hard fork of the jteeuwen/go-bindata hard fork that
was replaced and diffed the code against a Dec 1, 2015 copy of the
original repository we had as a cross-check of that hard fork.

This replaces references to jteeuwen/go-bindata to point to the
HashiCorp fork.

Also install new porter dep

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...