[Backport 2024.2] feature(dependabot): scan docker based loaders for updates

since dependabot is expecting very specific format of yaml files it can find and update:

name starts with values_
docker image need to be under image key

since we use same dockerhub repo for most of the loader images and dependabot only handle update of one of them, we had to split it to multiple files that would be scan seperatly. sct_config would collect all of those back into the configuration in the same structure it was until this change

Testing

PR pre-checks (self review)

[x] I added the relevant backport labels
[x] I didn't leave commented-out/debugging code

Reminders

Add New configuration option and document them (in sdcm/sct_config.py)
Add unit tests to cover my changes (under unit-test/ folder)
Update the Readme/doc folder relevant to this change (if needed)

(cherry picked from commit 50020224662a89759386ce03310af152e30c5eaa)

Parent PR: #9291

Cherry-pick of 50020224662a89759386ce03310af152e30c5eaa has failed:

On branch mergify/bp/branch-2024.2/pr-9291
Your branch is up to date with 'origin/branch-2024.2'.

You are currently cherry-picking commit 500202246.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
    modified:   .github/dependabot.yml
    new file:   defaults/docker_images/alternator-dns/values_alternator-dns.yaml
    new file:   defaults/docker_images/cassandra-stress/values_cassandra-stress.yaml
    new file:   defaults/docker_images/cdc-stresser/values_cdc-stresser.yaml
    new file:   defaults/docker_images/cql-stress-cassandra-stress/values_cql-stress-cassandra-stress.yaml
    new file:   defaults/docker_images/gemini/values_gemini.yaml
    new file:   defaults/docker_images/harry/values_harry.yaml
    new file:   defaults/docker_images/kcl/values_kcl.yaml
    new file:   defaults/docker_images/latte/values_latte.yaml
    new file:   defaults/docker_images/ndbench/values_ndbench.yaml
    new file:   defaults/docker_images/nosqlbench/values_nosqlbench.yaml
    new file:   defaults/docker_images/scylla-bench/values_scylla-bench.yaml
    new file:   defaults/docker_images/ycsb/values_ycsb.yaml

Unmerged paths:
  (use "git add <file>..." to mark resolution)
    both modified:   defaults/test_default.yaml
    both modified:   sdcm/sct_config.py

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

scylladb / scylla-cluster-tests