lava: Fix submission by fixing the metadata format

It turns out that LAVA really wants strings for the metadata entries and didn't like nesting a dictionary there.

For this reason it was failing to submit jobs with this error:

Submitting job group-apparmor-tpl.yaml: <Fault 400: "Problem with submitted job data: expected str for dictionary value @ data['metadata']['source']">

Unfortunately our custom lava-submit.py was silently eating the error so the pipeline was marked as successful.