Refactor configuration of remote services.

Configuration of remote services is now configured in a few variables:

• swh::remote_service::$service::config: default read-only configuration
• swh::remote_service::$service::config::writable: primary, writable copy of $service
• swh::remote_service::$service::config::$instance: instance of $service.

Locations should override the value of swh::remote_service::$service::config if
they have a local caching read-only copy.

All service configurations are modified to use the read-only/writable variables
consistently.