Unable to retrieve Tag name using pre Defined Variables or by other means

## Issue
Unable to retrieve the tag name used to trigger the build. Seams like there is not a specific variable that contains such information [https://github.com/jenkinsci/gitlab-plugin#defined-variables](url)

### Context
- **Gitlab plugin version**: 1.5.17
- **Gitlab version**: 12.1.8 (CE)
- **Jenkins version**: 2.222.4
- **Job type**: Pipeline

### Problem description

_Describe your problem in a meaningful way_:
1) Create/Push an new tag to one of my gitlab repos
2) The Jenkins Build gets triggered, as expected.
3) Retrieve the tag name that triggered the build

As far as I know the Gitlab webhook payload exposes the tag name under "ref". https://docs.gitlab.com/ce/user/project/integrations/webhooks.html