Pass "Pipeline started" and "Pipeline shutdown complete." log messages to WARN level

Presently, by default (warn level), in logstash logs, there is no information to say when logstash is started and when it is stopped.
If we activate --verbose option (to set info level), we have more than 7500 lines of logs just at startup, so it's too verbose for production needs.

My proposition to easily add this simple information in logs, is to pass "Pipeline started" and "Pipeline shutdown complete." log messages to WARN level.

Thereby, we could have such logs, by default :

```
{:timestamp=>"2015-11-05T18:39:24.391000+0100", :message=>"Pipeline started", :level=>:warn}
{:timestamp=>"2015-11-05T18:40:25.481000+0100", :message=>"Pipeline shutdown complete.", :level=>:warn}
```