Revisit the WgManager class

[CyrilBrulebois]

It might have been a mistake to have 3 mandatory parameters when creating a WgManager instance… those could be re-read from the existing config (if there's one), instead of having to be passed every single time.

See ffc9c16 which made this design issue obvious.

Similar issues might show up when the pirogue-admin server and its gRPC interface are implemented, that can be revisited at that time.