Packer builds VM for builder host

This sets up the infrastructure we'll use to build and package an app. 

By having a relatively boring virtual machine, we nudge ourselves in the direction of using Docker containers for the actual build process. This keeps our builders somewhat deterministically reproducible (vs. installing the packages by hand and forgetting the ones we really needed).

Admittedly we could use a tool like Ansible or shell scripts to install the packages, but these don't solve the problem of *removing* old state; which is a strong argument for immutable infrastructure.