Google Compute Builder

This is built on top of #691. The changes primarily are to facilitate testing. Some notes:

* Adopted a `Driver` interface instead of the API client. This interface allows us to very easily test all the logic of each and every step. 

* The tests actually found some bugs where we didn't test exit codes of the updates and bundling. So we do that.

* I made the steps and config public. I'm not sure why I do this, but I feel as though they should be usable as a package if they wanted to be (though no one does and I don't really recommend it).

* UI changes.

* Website covers how to get auth setup.

That should do!

/cc @kelseyhightower