Closes T646

The check_config method allows a dynamic check of the configuration for
a running storage. We can make sure that we have proper permissions on
the object storage as well as the database before running things.

If you want the archiver to have cloud abilities, install the
package python3-swh.objstorage.cloud on the server as well.

Actions storage:
- Open cache_content_get to retrieve contents in cache
- sql/upgrades/080: Add stored procedure to read contents from cache

Actions archiver:
- d/control: Add dependency to archiver on python3-swh.objstorage.cloud
- Renamed Archiver(Director|Worker) to
ArchiverWithRetentionPolicy(Director|Worker)
- Add ArchiverToBackend(Director|Worker)
- Add new celery task dedicated for new workers
- Update docstring details

Related T555

Closes T460

Closes T404