Add plugin spec

Whilst I've been trying to get more test coverage of some of my plugins, I found some nice ways of testing some common vagant plugin stuff. Just thought I should add it to vagrant-omnibus! :+1: 
This `require` should not be moved here. It is [supposed](https://docs.vagrantup.com/v2/plugins/development-basics.html) to be lazy loaded only if needed, i.e. only if the plugin is really triggered by Vagrant.