Refactor python code, build gnunetrest docker image and test it with an external gnunet image

This is the branch in which i told you i was beautifying your code :) I replaced your example file by proper tests. I also removed the gnunet docker creation since those sources should be in gnunet repo and we can just use it. And i'm now also building an image and container only with the gnunet rest stuff, to be able to test it together with the gnunet container.

You can pull the same image that gitlab builds from https://gitlab.com/juga0/gnunet-docker/container_registry

I've left intentionally python tests that do not pass. I don't why, but we should fix them. In gitlab pipeline, you can see tests not passing here: https://gitlab.com/juga0/gnunet-docker/-/jobs/830242316#L814, You can see the tests not passing locally, either by running docker-compose up, or launching a gnunet container exposing to your host the rest port and running inside of the python code pytest-3 tests (or pytest if you're in a virtualenv).

I'd recommend you look at the changes commit by commit, since they're quite a lot.

Ask me anything isn't clear ;)