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Updating log4j.xml to add default log file writing to replace the ">> ${LOGFILE}" line in the graylog2ctl script

First Commit:
- Adding new appender graylog2 with class name org.apache.log4j.DailyRollingFileAppender
- Setting log filename to /var/log/graylog2-server.log (Path debatable)
- DatePattern set to '.'yyyy-MM-dd
- Setting appender for loggers root and org.graylog2 to new graylog2 appender
- Changing level from warn to info

Second commit:
- Reset class name from DailyRollingFileAppender to simply RollingFileAppender
- Adding a rollingPolicy block:
- - ActiveFileName parameter value is /var/log/graylog2-server.log
- - FileNamePattern parameter value is /var/log/graylog2-server.%d{yyyy-MM-dd}.log.gz
To rotate files every night and compress them
- Removing file and DatePattern parameters