Add support for provisioner-less base boxes

Idempotently ensure the desired version of the Omnibus Chef MSI is installed. We install Chef during the `run_command` stage so we can use the sandbox to transfer `install.ps1` (basically a Window's version of `install.sh`).

Tested with a Chef-less box produced from @misheska's Packer templates:
https://github.com/misheska/basebox-packer/tree/master/template