Add a pre-up trigger to handle downloading the images tarball

These changes add a "get our images disk image" script, and a pre-up trigger
to invoke it.  This script is invoked on "vagrant up"; it checks for the
presence of an existing disk image (images.vdi) and downloads one if it is not
found.

This allows a "vagrant up" to Just Work the way people expect.


*Note:* This is just a start, a proof-of-concept.  The
[vagrant-triggers plugin](https://github.com/emyl/vagrant-triggers) is
required, and can be installed with:

```
vagrant plugin install vagrant-triggers
```

**Rationale**

This change is aimed at people -- like me -- who expect to have "vagrant up"
Just Work, and are a little leery of needing to do things outside of that.  I
also don't want to do anything involving "sudo" if my purpose is to just "give
it a whirl".

...and really, that's the whole point behind VM's and Vagrant, right?  To be
able to easily spin up environments with, ideally, outside of resource
consumption, no impact or risk to the host.  Which is probably your laptop...
That you do everything on.

If you're devopsy and looking at panamax, you're probably the sort of person
who doesn't deploy on Fridays, doesn't trust a single set of backups, and
won't allow anything to touch your laptop / primary workstation in any real
way without seriously vetting it first.