GitLab

These use libnrm/downstream-refactor and should be updated once that
branch is merged.

This commit pins the integration tests. This is done in the .integration.nix
file, which in turns uses the derivations from the 'argotest'
repository. These pin argopkgs - which itself pins nixpkgs, so
everything should be completely reproducible.

The integration target provisions the dependencies using nix-shell and
runs the basic 'hello world' container test.

Added integration tests to the `.gitlab-ci.yaml` file. These tests
run any runner tagged "integration". They depend on the 'argotest'
repository master branch being able to run the tests, which
uses 'argopkgs' repository. The runner must be Nix
enabled. This therefore currently uses a rolling release of the
integration tests.

Since our tests include checking that coolr can access the rapl
interface, we should ensure that only runners having rapl will test the
code.

No need to trigger CI for things that are not ready.

flake8 is an automatic style checker. This is very useful to ensure that
we have a consistent style as more people start contributing to the
code.

We ignore coolr for now, as it's a big offender.

Gitlab CI support, for a py.test run inside the pipenv configuration.