Automatic merge from submit-queue

kubeadm: Refactoring the apiconfig and addons phases 

**What this PR does / why we need it**:

First commit: Fix the defaulting for AuthorizationMode - **Ready for review**
Second commit: Refactoring the apiconfig and addons phases in kubeadm - **work in progress, broken at the moment**

**Special notes for your reviewer**:

Please take a look at the first commit now. You can also see the direction I'm going with the addons and apiconfig phases.

For example, I'm using Go templates instead of creating native Go structs for kube-dns and kube-proxy now. The question is if I should do it for the RBAC rules as well, it might make sense there as well.

Converting the dns addon to a yaml spec makes it 100x easier to maintain when changes are made to the "upstream" DNS spec, and also more swappable. We could for instance have a configuration param for it for those who want a custom file, etc.

**Release note**:

```release-note
NONE
```

@mikedanese @pires @lukemarsden @errordeveloper @dgoodwin @liggitt @andrewrynhard