Improved logging in agent:

- when there is an external log service present we can use, use it instead
  of our "internal" log service. If no log service is present, we can always
  log to our internal service instead;
- when no valid configuration property is given for the log handler, do not
  fall back on the global default, but use the current setting instead. This
  way, we can avoid unwanted toggling of log levels when updating the agent's
  configuration.



git-svn-id: https://svn.apache.org/repos/asf/ace/trunk@1582291 13f79535-47bb-0310-9956-ffa450edef68