Simplify values interpretation in charts/sylva-units/templates/_os-images.tpl

The following discussion from !2401 (merged) should be addressed:

• @baburciu started a discussion: (+2 comments)

  nit: from what you pointed out at !2248 (comment 1943671431), we could probably have first

  {{- $envAll := set . "Values" (include "interpret-values-gotpl" . | fromJson) -}}

  for both images_from_os_images and check_all_images_override_exist named templates to avoid the various interpretation templates calls