don't define cluster.(capo|capm3).os_image_selector as a templated value
This MR complements/fixes !153 (merged).
With !153 (merged) alone, the settings can't be used from in sylva-core, or else we hit the issue described in sylva-projects/sylva-core!3702 (comment 2360380245) which is due to the fact that cluster.⋮.os_image_selector can't be a templated value, because it's used by find-cluster-image-selectors before interpretation occurs and needs to be a dict at that stage (sylva-projects/sylva-core!3693 (diffs)).
This issue wasn't catched when !153 (merged) was merged, because it was tested with sylva-projects/sylva-core!3702 (merged) which was the sylva-core MR meant to start using os_image_selector, but sylva-projects/sylva-core!3693 (merged) which also uses id indirectly, merged in the meantime.
This MR was tested with a revised version of sylva-projects/sylva-core!3702 (merged) (successfully)