Add mysqld_exporter and podman-exporter image overrides

[vyzigold]

Add ceilometerMysqldExporterImage and edpmPodmanExporterImage fields to the update_containers template. This allows Zuul content-provider jobs for mysqld_exporter and prometheus-podman-exporter to override the container images used in functional tests via the OpenStackVersion custom resource.

Generated-By: Claude-Code claude-opus-4-6

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