config: complains when a setting is not tracked

config: complains when a setting is not tracked

* not all config items are tracked, so it does not take any effect after
  we sucessfully changed them using "ceph tell <daemon> injectargs  --foo-bar 15',
  as shown by the command output:
    $daemon: foo_bar = '15'
  if foo-bar happens to be the one not tracked by any components in <daemon>.
  in this fix, the message of
    $daemon: foo_bar = '15' (unchangeable)
  is returned instead. nevertheless, the config is still updated. as
  "ceph daemon <daemon> config show | grep foo_bar" shows:
    "foo_bar": "15"
  this helps user to understand that the setting is not dynamically
  changeable.
* update the test accordingly

Fixes: #11692
Signed-off-by: Kefu Chai <kchai@redhat.com>