automatic OS image activation (capm3)

This issue tracks the work related to sylva-projects#43 (closed) item 2 "automatic OS image enabling" for the sub-case of CAPM3.

Recap of the MRs, roughly in the order in which they'll merge:

• os-images-server: introduce OS image selectors (!3668 - merged)
  • introduce OS image selectors (sylva-projects/sylva-elements/helm-charts/os-image-server!107 - merged)
    • needs add helpers for OS image selection (sylva-projects/sylva-elements/helm-charts/sylva-library!44 - merged) which is common with #2013 (closed)
• os-images-server: clone into workload clusters ... (!3936 - merged)
  • needs add an output configmap with information about ... (sylva-projects/sylva-elements/helm-charts/os-image-server!109 - merged)
• os-images-server: define OS image selectors == ... (!3693 - merged)
  • needs OS images helpers: add find-cluster-image-selec... (sylva-projects/sylva-elements/helm-charts/sylva-library!81 - merged) which is common with #2014 (closed)
• code simplification/cleanup: use sylva-library helper for OS image PV size c... (sylva-projects/sylva-elements/helm-charts/os-image-server!106 - merged)
• os-images-server: simplify how we restrict which images are downloaded on bootstrap os-images-server: simplify how we restrict whic... (!3728 - merged)
• simplify OS images activation (os_images/sylva_... (!3794 - merged) (common with ~#2013 (closed))

📑 there is also a documentation portal MR related to this change and related ones: sylva-projects.gitlab.io!144 (merged)