plugin XEP-0373: minor renaming + set `gpg_provider` in client: - `sat` and `self.__sat` have been renamed to `host` and `self.host` for consistency with other plugins. - `gpg_provider` is set in client while profile is connecting, so the instance is easily accessible to plugins depending on XEP-0373 rel 381

this certificates are used to enable TLS during tests. Those must be the same as in
backend_e2e (they are generated there)