Added the `maxBackupIndex` option to the default log appender

See http://logging.apache.org/log4j/1.2/apidocs/org/apache/log4j/RollingFileAppender.html#maxBackupIndex

Closes #192
Fixed the "maxBackupIndex" support

* Renamed `maxbackupindex` to `max_backup_index`
* Fixed the ERb syntax to not mess up the YAML format and indentation
* Use the same "maxBackupIndex" also for slowlogs

Verified with Vagrant.

Related: #192