mgr/dashboard: Add whitelist to guard

mgr/dashboard: Add whitelist to guard

After PR https://github.com/ceph/ceph/pull/26572, when RGW is not
  configured, accessing /rgw drop-down (daemons, users or buckets)
  results in nothing apparently happening (not even an error).

  Under the curtains, what is happening is that the ModuleStatusGuard
  has redirected the route to the rgw/501, but as this route is now
  under parent rgw route handler, which sets CanActivateChild guards,
  this results in a new ModuleStatusGuard invokation, a subsequent
  failure and a new redirection to rgw/501.

  Several approaches could be taken here:
  - Remove error pages from lazy-loaded modules. Probably it does not
  make sense to have a 501 page per component.
  - Add some whitelist to avoid this kind of loop (e.g.: 501, or any
      error page).
  - Set a max number of redirections (cautionary measure).

Fixes: https://tracker.ceph.com/issues/39125
Signed-off-by: Ernesto Puerta <epuertat@redhat.com>