config: Remove default config values for backends

These default config values are mostly irrelevant for any kind of
deployment. Removing the default values make it clear and mandatory
these need to be explicitly configured.

This also prevent the swh.web.utils.archive module from
executing (backend instantiation) code at module loading time.

Introduces a new patch_backend fixture to help mocking a specific method
of a known configured backend (storage, vault, search etc.) to try to
provide a standard and unified process to mock these in all tests.