Avoid the creation of log4j routing appender for log events without the `pipeline.id` fishtag.
In this way no spurious log file named "pipeline_${ctx:pipeline.id}.log" and logs are not duplicated with main Logstash log file.
Remove an useful dynamic creation of appender's log file which leverages the `log.format` property
also when it's explicit by the appender itself.
Log4j configuration leverages the placeholder `${sys:ls.log.format}` to compose the name of the log file.
This generates some not evident conflicts in log4j internals, these conflicts became evident when enabling the `pipeline.separate_logs` feature is enabled and the log4j appender definitions contains both json and plain format.
The problem is that under those circumstances the rollover of the log file doesn't happen.
This commit also add a test against the production log4j configuration, to avoid future regressions.
