Redesign of logging.

Goal: make logging infrastructure setup usable in production.

- Replace public logstash-forwarder-cert with on-the-fly generated ones.
- Introduce internal config-server to serve certs.
- Introduce digitalwonderland/logstash-forwarder to forward *docker*-logs.
- Bumping to a koha-salt-docker without logstash-forwarder.

Issues remaining:
- Very poor parsing of logs now. Should improve logstash.conf.
- Actual deployment to prod environment untested.