GitLab

This commits adds a gitlab-ci integration test target, and the
.integration.nix file, which specifies the exact test version pinning to
use.

Judicael's code, unchanged, extracted from the master branch of pre-ECP
NodeOS repository.

This is not a filter-branch, just a cp -r. We are not losing anything
important in the git history, as it was only 3 commits long, without any
details on the history of the code.