Add logback config

[yanokwa]

Tested by running on ./aggregate-cli -i -f -y -c /root/aggregate-config.json on https://test.aggregate.getodk.org.

• Confirmed that upgrade was successful
• Confirmed that no debug information was printed out
• Confirmed that output was printed to screen but not the log file (is this expected behavior, @ggalmazor?)

Once this is reviewed and merge, I can do a release.

[lognaturel]

The behavior looks expected to me. There would only be a file created if you hit info-level logging from the tool itself and you probably didn't.

[ggalmazor]

Confirmed that output was printed to screen but not the log file (is this expected behavior, @ggalmazor?)

Yes, that'd be correct. The intention was to use standard output/error for user feedback. Errors and other tracing info should go to the logs.