WIP: Re-implementing kube-up in go

This is very much a WIP. Wanted to get comments before I get too far down any rabbit holes.

kube-up.go follows the structure of kube-up in util.sh pretty closely. My next step is to take the library stuff currently in kube_updown.go and fit it nicely into the cloudprovider abstraction.

Go documentation for a function should start with the name of the function and end with a period. In this case, it should read something like "AddMaster adds the cluster's master instance.".