Retrieve GitlabHook data in upstream builds if available

[Kimamisa]

This patch allows downstream jobs to use the gitlab plugin features, such as accepting a merge request, adding a note, but also populating the environment with the gitlab data.

[Kimamisa]

FYI, we've been using it for almost a year now, works perfectly for us.

[Kimamisa]

rerun

[splisson-altair]

+1

[n0rig]

+1