improve get_image_refs.py to use helm get manifest/hooks
Summary
After some manual tests to deploy sylva with a "prepopulated registry", we observed sometime failed Helm releases probably due to missing container images used in hook job.
I say "Probably", because as most hook config is hook-delete-policy=hook-failed the failed resource is deleted, so there is no trace of it on the remaining resources
The only way to manage this issue is to parse helm get hooks <helmrelease>
and we can parse also helm get manifest <helmreleas> as algorithm is the same.