Support for including Jenkins job in gitlab-ci.yml configuration

This plugin works really well when you only have a Jenkins build job. But we are running both Jenkins and `gitlab-ci` build parts (due to legacy mostly), and found it very problematic to mesh the Jenkins job into the GitLab CI configuration. For example, no matter what you name the Jenkins job/stage, it doesn't seem to be possible to make it part of the CI workflow.

Scenario:

In GitLab CI, we want to have the stages `Lint`, `Test`, and `Deploy`. Jenkins builds the project, so we would want the Jenkins job to only trigger after `Test`, but placing the name of the Jenkins job in the stage list doesn't affect its execution (which makes sense, as webhooks are different from CI pipelines).

A possible solution I could see is somehow supporting calling the Jenkins webhook from the CI pipeline, but I don't know where the special GitLab variables the plugin needs come from, exactly.