Major overhaul of the slapd (OpenLDAP) image.

- Use a single volume for the config and data LDAP directories, easier
to keep in sync in different cases (backups, migration).
- Add backup script, save to backup volume.
- Allow passing the location of the SSL key and cert to enable SSL,
otherwise a self-signed certificate and key are generated with the FQDN
of the container.
- Drop the dockerfile-lint linter, didn't give any useful advice.
- Change base image to Debian Buster.
- Set stopsignal for the container.
- Allow setting the log level.
- Unset the cleartext root password environment variable.