metrics-server: try replacing helm with kustomize

[waynr]

This PR proposes the use of the kustomize included in kubectl instead of
the third-party helm to deploy metrics-server.

The way this works is roughly:

1. create a DOKS cluster (just like before) 1. clone
   kubernetes-incubator/metrics-server at a specific git ref 1. call
   kubectl kustomize in the metrics-server directory

• this makes use of the kustomization.yaml file in the metrics-server
  directory
• the kustomization.yaml file points at example deployment manifests in
  the cloned kubernetes-incubator/metrics-server directory
• the kustomization.yaml patches the upstream example deployment
  manifests with settings similar to what came with the helm chart 1. just
  like before, wait until the metrics-server pod(s) are ready

One major difference from the helm chart is that with helm it was easy
to deploy metrics-server to its own namespace whereas the upstream
example manifests on which the kustomize approach is based assume
deployment in kube-system. It wasn't straightforward to use kustomize
patches or bases to do something similar -- I'm sure it's possible, I just
haven't found the minimal set of patches necessary yet.

[waynr]

I opened this PR because it looked like circle ci wasn't running against #9, but as soon as I opened this I saw the circle ci checks appear on that PR so I'm gonna close this one.