As of VirtualBox 6.1.28, host-only networks are restricted to
`192.168.56.0/21` by default, so spinning up the Vagrant boxes with
secondary networks fail with an error `E_ACCESSDENIED`.

Update the IP configuration of the Vagrant boxes to fall within the
allowed range so that we don't need to ask developers from the
community to make VirtualBox configuration updates outside of their
normal Vagrant use.

This replaces all usage of `protoc` with `buf`. See `tools/buf/README.md` for more.

We set the Eval field on job (de-)registration only after all servers
get upgraded, to avoid dealing with duplicate evals.

Rkt has been archived and is no longer an active project:
* https://github.com/rkt/rkt
* https://github.com/rkt/rkt/issues/4024

The rkt driver will continue to live as an external plugin.

Avoid Vagrant/virtualbox interferring with host audio when the VM boots.

By having an ip address by default, developers can access any service on
the Vagrant linux box without needing to setup port maps.

Also tease out some dev and docker bits from Vagrant scripts to ease
sharing with Docker script

Freebsd 11.1 went EOL 9/30/18:
https://lists.freebsd.org/pipermail/freebsd-announce/2018-September/001842.html

Was seeing a failure with shared object "libdl.so.1" not found which
is required for bash

Set shell to sh which is present by default on freebsd. Also set
networking to host dhcp due to limitations with NFS sharing:
https://github.com/hashicorp/vagrant/issues/9063

Use single `grep` with `^` condition instead of `grep` + `cat` + `wc`

Technically just the true->false is needed. The `chown -R` was a first
attempt that didn't fix the bug, but it's a nice simplification.

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`.

  * Change the GO_VERSION to match the correct upstream version from
    (https://storage.googleapis.com/golang/)

Had to use the hack from https://github.com/golang/go/issues/12443