mgr/dashboard: Drop invalid RGW client instances, improve logging
- Log RGW settings for easier problem tracking.
- Drop RGW client instances that do not exist anymore. This happens when a RGW service is removed via the orchestrator. The Dashboard tries to access the RGW client using the previous settings which leads to an error which might be confusing. Without dropping the error message 'RGW REST API failed request ...' is displayed instead of the correct one 'No RGW found'. Dropping the RGW client instance will produce correct error messages the next time the backend tries to establish a new connection.