Add a swh.vault.api.wsgi module to instanciate the (singleton) wsgi app object

Typically used by external wsgi server like uwsgi or gunicorn, e.g.

gunicorn swh.vault.api.wsgi:app

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

Make it possible to specify the config file via SWH_CONFIG_FILENAME env var

so that we have a chance to explicitely choose the config file used when
running the wsgi app using a wsgi server like gunicorn.

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).
• Extract instanciation (thus confguration) of remote/external services (storage, cache, scheduler) from the VaultBackend constructor; these are now passed as init arguments; their instanciation/configuration is thus now handled by swh.vault.api.server and swh.vault.cooker.get_cooker (with some support in swh.vault.get_vault).

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

pifpaf is not needed any more.