Preprocessor modularization

See merge request !15

# Conflicts:
#   .gitlab-ci.yml
#   README.md

Fixing testing

See merge request !20

Fixtures for connection and test data
Using dotenv to load env vars

 - proper escaping
 - wrong XML tag

Adding preprocessor config file

The current production docker verison or configuration only allows to
forward a port either by using udp or tcp, not both (even though that's
what is suggested by fluentd).
docker will use the last entry and silently delete the former.

This is not a problem for us since we only talk to fluentd via tcp.

(for consistency, it's not strictly necessary)

This would only be a problem if fluentd wasn't running, but
since fluentd doesn't log to fluentd, kibana wouldn't help anyway.