Add sample for OpenShift's DNS operator to resolve AD-Zone

[ibotty]

This issue is more of a stopgap until proper documentation is written for people searching for that information. I intend to write this up properly.

OpenShift's DNS operator does not allow editing the coredns configmap when in managed state. It does support changing the CRD though. The following file does the same on OpenShift as the file in tests/files/coredns-snippet.template.
The file can be applied with

oc patch dns.operator/default --type merge --patch-file /path/to/file

(Of course AD_SERVER_IP has to be the actual IP.)

spec:
  servers:
  - name: ad-zone
    zones:
    - ad.schaeffer-ag.de
    forwardPlugin:
      upstreams:
      - AD_SERVER_IP