tools/get_image_refs.py: validate rules files, fix rule file

Benjamin Le Diguerherrequested to merge
patch_get_image_ref_with_image_to_unit_mapping into main

This MR fixes a runtime error that happened when the YAML config (tools/unit_mapping_rules.yml) had commented-out or badly formatted rules.*

Closes #2556 (closed)

Edited by Thomas Morin

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