Use ansible.builtin.stat to check src file existence

[cjeanner]

ansible.builtin.exists is a local test only, and therefore doesn't
work against remote files.

This change modify the way we check for source values.yaml availability,
allowing to run against a remote host (mostly for coming testing)

As a pull request owner and reviewers, I checked that:

• Appropriate testing is done and actually running

[github-actions]

Thanks for the PR! ❤️
I am marking it as a draft, once passing your happy and the PR is passing CI click the "Ready for review" button below.

[softwarefactory-project-zuul]

Build failed (check pipeline). Post recheck (without leading slash)
to rerun all jobs. Make sure the failure cause has been resolved before
you rerun jobs.

https://review.rdoproject.org/zuul/buildset/766984a7967c48abaad458e05c34a13d

❌ openstack-k8s-operators-content-provider MERGE_CONFLICT in 6s
⚠️ podified-multinode-edpm-deployment-crc SKIPPED Skipped due to failed job openstack-k8s-operators-content-provider
⚠️ cifmw-crc-podified-edpm-baremetal SKIPPED Skipped due to failed job openstack-k8s-operators-content-provider
✔️ noop SUCCESS in 0s
❌ cifmw-pod-ansible-test MERGE_CONFLICT in 0s
❌ cifmw-pod-pre-commit MERGE_CONFLICT in 0s
❌ cifmw-molecule-ci_gen_kustomize_values MERGE_CONFLICT in 1s

[rebtoor]

/approve

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

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