Implement pull secrets

[schnatterer]

For all pods except Jenkins, SCMM, Registry, ArgoCD

We have different options for using them in airGapped envs: Cloudogu
Ecosystem and Argo CD Operator (WIP) and an external registry.

Introduces new config/ CLI:

• --create-image-pull-secrets
• --registry-username-read-only and --registry-password-read-only': least privilege: if you don't want to use --registry-username and -password as pull secrets, that allow for writing to the registry. The cluster only needs to read.

More or less unrelated changes:

• Introduces Content class, which we already know will in future hold example apps and exercises so they can be switched on and off separately. Also will improve maintenance of the huge ArgoCd.groovy monolith.
• missing image params and config options
• migrates some properties from pre-templating times from groovy to values (in Mailhog, Vault, External Secrets Operator)
• Changes Argo CD applications to use valuesObject instead of values (String). This improves visualization and simplifies debugging.

How to test

See https://github.com/cloudogu/gitops-playground/blob/58fdd0/docs/developers.md#proper-test

After everything it built and deployed there should be

• no degraded app in argocd
• no failing pods returned by k get pods -A
• and this should only return localhost:30000 pods:

$  kubectl get pods --all-namespaces -o jsonpath="{.items[*].spec.containers[*].image}" \
  | tr -s '[[:space:]]' '\n' | sort | uniq \
  | grep -v 'harbor\|jenkins\|scmm\|rancher\|argo\|redis\|dex'

localhost:30000/proxy/eso:latest
localhost:30000/proxy/grafana:latest
localhost:30000/proxy/ingress-nginx:latest
localhost:30000/proxy/k8s-sidecar:latest
localhost:30000/proxy/mailhog:latest
localhost:30000/proxy/nginx:latest
localhost:30000/proxy/prometheus-operator:latest
localhost:30000/proxy/vault:latest

[schnatterer]

For the reviewer:

• Please take extra care to check if I didn't forget any negative tests: without the imagePullSecrets param there should be no imagePullSecrets field in the helm chart.
• What do you think about the design choice of FeatureWithImage in general and as a trait?