Add testing through GH Actions

[rbott]

We should add at least some sort of testing step to validate Pull Requests through github actions. Adding unit tests will probably require quite some refactoring and yield only small result.

I would instead vote for an end-to-end test as a start.
This would include the follwing steps:

1. start a ganeti vcluster (see this repository)
2. run prometheus-ganeti-exporter
3. validate that data from the vcluster is exposed

This way we learn at least if the code is somewhat working. Any opinions on this?

[rbott]

This PR will create basic Docker images to run pylint against the current codebase. Once this has been merged, I will open a PR with a GH Action step to this repository so we get at least a very basic check of the code.

We can improve from there.