OIDC RP Logout: Fixed issues during testing

- Disabled by default due to strict rejection by auth systems.
- Fixed issue when autoloading logout URL, but not provided in
  autodiscovery response.
- Added proper handling for if the logout URL contains a query string
  already.
- Added extra tests to cover.
- Forced config endpoint to be used, if set as a string, instead of
  autodiscovery endpoint.