* Allow the logback directory for ETL jobs to be overridden using system property.
See https://logback.qos.ch/manual/configuration.html#variableSubstitution for more details.
* Add WHZ_ETL_TEMP_DIR env var and play config to control where the ETL job logs & temp files to be saved.
This enables us to move away from the default /var/tmp/wherehows directory.
- Move logback.xml in metadata-etl to etl_logback.xml under backend/conf to avoid multiple logback config in classpath. ETL jobs are able to write to their own log file again.
- Replace generated single string command with String[] and invoke Runtime.getRuntime().exec(String[])
