Update logging for graylog.

[Foxcapades]

This may need to be moved to the core lib.

May be as simple as introducing and configuring https://github.com/mp911de/logstash-gelf/ for our log4j2

[rbelnap]

Note that we are not directly logging to graylog, so I don't think gelf will be much help - Docker logs stdout which is then routed through fluentd, then into graylog. Fluentd in this case will be doing the parsing of the logs... it is very flexible, as long as there is a standard format. https://docs.fluentd.org/parser if you're curious. Logging as json may be the easiest for machines, but could be an issue for humans.