Support detecting merge request from CHANGE_ID variable

### Context

- **Gitlab plugin version**: 1.5.13
- **Gitlab version**: 11.10.4
- **Jenkins version**: 2.190.3
- **Job type**: Multibranch pipeline

### Problem description

I use this plugin in tandem with [gitlab-branch-source-plugin](https://github.com/jenkinsci/gitlab-branch-source-plugin) to support merge requests on multibranch pipelines.

However, I would like to leave a comment on the merge request as part of the pipeline but `addGitLabMRComment` doesn't work for multibranch pipelines.

`gitlab-branch-source-plugin` populates the `CHANGE_ID` environment variable with the merge request ID as per [these docs](https://jenkins.io/doc/book/pipeline/multibranch/).

It would be very useful if this plugin could use `CHANGE_ID` to detect the merge request and allow `addGitLabMRComment` to be used.