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)
2. clone kubernetes-incubator/metrics-server at a specific git ref
3. 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.

[snormore]

I just merged a PR that switches to CircleCI so you shouldn't hit the invalid-token issue anymore with this after rebasing onto master.