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`).
Allow `require_chef_omnibus` to be set to false
Merge pull request #2 from schisamo/install-chef

Add support for provisioner-less base boxes