Add an ability to not send targetUrl when updating pipeline status

At the moment, `updateGitlabCommitStatus` pipeline step seems to always send `targetUrl` of the jenkins project to the gitlab.

However, recently gitlab seems to have started validating the target URL. If it is not reachable, status update never suceeds.

Our jenkins is only accessible in the internal network. That means that `targetUrl` is not reachable from Gitlab and thus status update always fails.

Could you add an optional parameter to `updateGitlabCommitStatus` to not send target url with build status? Or maybe make it global per-gitlab-connection setting? Would you be fine with Pull Request for this?