Normalize storage and objstorage configuration schema

For the first time, we need to be able to deploy a mixed swh-storage. So far, we had only on swh-storage (swh's db + local objstorage as pathslicing).

The use case is the loader-svn test working with a dedicated db and our standard objstorage (accessed remotely so not a pathslicing one). As of now, the objstorage for our storage is hard-coded to a pathslicing. So, impossible in its current state to setup.

Also, it turns out that our current objstorage and storage configuration are not yet unified. This, in effect, limits the previous steps (we cannot express a different objstorage with the current storage configuration scheme). So we need to also overcome this.

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Migrated from T613 (view on Phabricator)