[change] Rename deafult value of OPENWISP_RADIUS_PASSWORD_RESET_URLS to __all__

[nemesifier]

In openwisp/openwisp-monitoring#376 we have used a different dict value for the fallback value, instead of default we have used __all__ which may be more explicit.

I think it's a good idea to change it here too, while maintaining backward compatibility and still support default too, but the documentation should mention only __all__.