Add example of using kustomization.

[gtcno]

Example of using https://github.com/kubernetes-sigs/kustomize to have variant deployments to dev and prod.

The choice of using this project is that it will be integrated into kubectl as a subcommand (kubernetes/enhancements#633).
But for now, it requires a stand alone binary(https://github.com/kubernetes-sigs/kustomize/releases)

The flow would be as follow:

run kustomize build nais/dev or kustomize build nais/prod to generate specifications for dev and prod.
kustomize will first apply configuration + rules to the yaml files in the base folder, then the rules + configuration in the dev/prod folders are applied to generate the final specifications.

In my opinion

Pros:

• Pure yaml file no templating is required, which makes it easier to work with as well as easier to read.
• kubectl support in the near future.
• Reduces boilerplate or copy and paste to a certain degree.

Cons:

• Not as simple or easy to read as having a dev and prod yaml file.

Thoughts?

[geiralund]

I do like this- and would prefer this rather than specify envs in vault.