bridge: bridge can now be set using environment variable: The `LIBERVIA_BRIDGE_NAME` environment variable can now be used to specify the bridge to use. If set and different from empty string, the environment has precedence over config file value. For `pb` bridge, the following environment variable can also be used: - LIBERVIA_BRIDGE_PB_CONNECTION_TYPE - LIBERVIA_BRIDGE_PB_HOST - LIBERVIA_BRIDGE_PB_PORT