Cleanup MetaElement local state
Description
MetaElements are kept in local caches in multiple place. Removing them allows to save around 400MB when run against debian-stack.
Changes proposed in this merge request:
- Only store exceptions when in testsuite, otherwise MetaElement don't get cleaned if there is a "stack" element, since it doesn't have a "stack.yaml" and this raises an exception
- Cleanup loader caches after we are done loading.
This merge request, when approved, will close: