Add possibility to run custom playbooks

This adds an option to run custom playbooks, that can be either copied or cloned with a submodule into the k8s-custom folder. Further details are included within the commit message.

Closes: #374 (closed)

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Merge Prerequisites

• Description and title are descriptive
• Code is readable and syntactically correct
• Code is understandable
• Tests added (if needed)
• Docs updated
• Commit messages look good

As a developer: please do not tick these boxes yourself. As a reviewer: please get yourself a hot beverage.