Refactor Vault a bit, mainly pytest, config and BaseDb-based VaultBackend

• Extract the main cli command in a dedicated cli.py module and make it an entrypoint

• Refactor the VaultBackend to use BaseDb and pool-based db access

• Normalize the configuration of VaultBackend and cooker

  ensure every service for which an (remote or local) access is needed uses the same config structure (dict with 'cls' and 'args' keys).

• Rewrite tests using pytest's fuxture and adapt them to recent refactorings

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Migrated from D1108 (view on Phabricator)