lister.cli: Clarify cli's configuration loading step

lister.debian: Actually use the db_engine passed to the hook function

The end goal being to actually be able to use it successfully (it did not work).

Test Plan

• tox
• Initialize a new db from scratch:

swh lister --db-url "postgresql:///?service=admin-staging-swh-lister" db-init

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Migrated from D2232 (view on Phabricator)