Improve the config-id parameter support

It seems there is something wrong with the config-id support on the cli.

For example, in production:

swh-scrubber=> select * from datastore;
  id  | package |   class    |                                               instance
------+---------+------------+-------------------------------------------------------------------------------------------------------
    1 | storage | postgresql | user=xxx password=xxx dbname=softwareheritage host=db.internal.softwareheritage.org port=5432
 4997 | storage | postgresql | user=xxx password=xxx dbname=softwareheritage host=somerset.internal.softwareheritage.org port=5432
(2 rows)

swh-scrubber=> select * from check_config where name ='check-config-directory';
 id | datastore | object_type | nb_partitions |          name          | comment
----+-----------+-------------+---------------+------------------------+---------
  9 |         1 | directory   |         16384 | check-config-directory |
 13 |      4997 | directory   |         16384 | check-config-directory |
(2 rows)

When a config-id is not provided in the command line the scrubber will silently choose the first datastore in cli.py and fail after the first call to get_datastore. It seems the integrity is very weak.

Is this config-id really mandatory or couldn't we always use the datastore instance connection string as an "entrypoint" of the configuration?