Refactor the database creation code and a test for the cli command

this comes with a few changes in the configuration file format to
normalize it a bit and ease testing:

• 'ps' config cls for the archive has been renamed as 'direct'
• 'ps' confif cls for the provenance db has been renamed as 'local' (similar to other swh packages),
• 'ps_np' has been suppressed in favor of a 'with_path' config flag in the 'provenance' section.

Note: this is only a first "normalization" step, further revisions will
come to use the swh.core.db scaffolding instead of the current ad hoc
code.