Default to env.STAGE_NAME for gitlabCommitStatus

## Issue

### Context
- **Gitlab plugin version**: 1.5.11
- **Gitlab version**: GitLab Community Edition 10.5.4 8d768f9
- **Jenkins version**: Jenkins 2.150.1
- **Job type**: Pipeline

### Problem description

Since `pipeline-stage-step-plugin` 2.3 was released we can get the current stage name with `env.STAGE_NAME`. See [JENKINS-44456](https://issues.jenkins-ci.org/browse/JENKINS-44456) for a description of the feature. I use this feature like this:

```groovy
pipeline {
stages {
stage('Lint') {
steps {
gitlabCommitStatus(name: env.STAGE_NAME) {
sh 'lint.sh'
}
}
}
}
}
```

I propose we let the `name` parameter to `gitlabCommitStatus` default to `env.STAGE_NAME` so that this pattern gets used by default. The reason is that this makes it easier to keep GitLab steps and Jenkins steps in sync.