Externalise image definitions

[achilleas-k]

Ordered list of tasks towards externalising image definitions (from Go code into YAML):

• Define package sets externally
  • Fedora Extract Fedora package sets into a single YAML file #1205
  • [in-progress-mvo] RHEL & CentOS
• Define base partition tables externally
• Define image config externally
  • Will probably require changing the ImageConfig structure to remove osbuild options.
  • Consider merging with OSCustomizations (this might not make sense in the end; needs more thought).
• Define supported customizations for each image type declaratively
  • Draft idea: Generalise and clean up blueprint customization validation for image types #1216
• Define image types externally

Please comment or even edit this issue to:

• Add more information about certain steps.
• Create and pick-up (self-assign) sub-tasks.
• Discuss the order.