Migrate GitLab Plugin for Jenkins to API V4
We want to ensure that GitLab Plugin for Jenkins will continue working even after the GitLab API v3 will be deprecated in %9.5.
In order to to that, we can create a PR to the original project with the changes and get in touch with the developers to have it merged into the official release. Users of GitLab with a 8.x version can continue using the actual plugin version.
cc @ayufan