Related T1508

by describing the config file loading nightmare.

for the config_file argument, rather than declaration time.

This is required so that if this function is called with None as first
argument (which is what happens in cli.py when user do not specify a
config file option).

Long story short, this fixes the rpc-server cli command: if a config file
is not explicitely given as option, fall back to default config file search
behavior instead of crashing.

so that one should use:

  swh vault rpc-server

as in other swh packages.

The deprecated swh-vault command should still work as expected.

also set the main command name to 'vault' so the main 'swh' cli tool
can generate shell completion properly.

for the sake of consistency.

so tests are not broken when next swh.core version is published.

Might as well, as we already depend on 0.0.56+.

this later does not brings anything over the default's one.

This requires swh.core >= 0.0.56

Related T1532

Prior to this commit, during the case of committing in another
branch (ref), there were dangling files in the HEAD branch.

pifpaf is not needed any more.

- 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).

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

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

  gunicorn swh.vault.api.wsgi:app

required to pull storage's test dependencies (like hypothesis).

related to T1330

Summary: Reported by Ludovic Courtès on IRC

Test Plan: make docs

Reviewers: seirl

Differential Revision: https://forge.softwareheritage.org/D674