Feature: Upgrade a Chef Omnibus Installation

Its a bit of a :frowning: to upgrade an already-bootstrapped node's Chef installation. You can use the [omnibus_updater](https://github.com/hw-cookbooks/omnibus_updater) cookbook, but it 
* doesn't support Windows MSI
* requires you to somehow (role, override, w/e) get it into your run-list.

The idea here is to add a new feature `upgrade_omnibus` which acts very similarly to the `uninstall_chef` functionality.

 :construction:

I'm passing this forward to @ivey and am going to leave some comments about what I've written so far.