Provide merge request labels as environment variables

[anton-johansson]

Closes #900
Also see #646

Example pipeline:

pipeline {
    agent any
    triggers {
        gitlab(
            triggerOnPush: true,
            triggerOnMergeRequest: true,
            triggerOpenMergeRequestOnPush: "source",
            triggerOnNoteRequest: true,
            branchFilterType: "All",
            skipWorkInProgressMergeRequest: false)
    }
    stages {
        stage('Prepare') {
            steps {
                echo sh(returnStdout: true, script: 'env')
            }
        }
    }
}

Output:

[Pipeline] sh
+ env
[Pipeline] echo
gitlabMergeRequestTargetProjectId=1
gitlabSourceRepoURL=ssh://git@localhost:10022/root/anton.git
JENKINS_HOME=/var/jenkins_home
gitlabTargetRepoHttpUrl=http://localhost:10080/root/anton.git
JENKINS_UC_EXPERIMENTAL=https://updates.jenkins.io/experimental
gitlabMergedByUser=root
RUN_CHANGES_DISPLAY_URL=http://localhost:8080/job/anton/5/display/redirect?page=changes
HOSTNAME=4ea3e1ce3cbc
gitlabTargetRepoSshUrl=ssh://git@localhost:10022/root/anton.git
SHLVL=0
NODE_LABELS=master
HUDSON_URL=http://localhost:8080/
HOME=/var/jenkins_home
BUILD_URL=http://localhost:8080/job/anton/5/
HUDSON_COOKIE=85dd3aca-ba51-4ed6-91ce-ffad4cb29895
gitlabTargetBranch=master
JENKINS_SERVER_COOKIE=durable-6b17982e7e0d911a16d9ced57c7aafff
gitlabMergeRequestIid=1
JENKINS_UC=https://updates.jenkins.io
gitlabSourceRepoHttpUrl=http://localhost:10080/root/anton.git
gitlabMergeRequestState=opened
WORKSPACE=/var/jenkins_home/workspace/anton
gitlabMergeRequestTitle=Update README.md
JAVA_VERSION=8u151
gitlabMergeRequestLastCommit=d0b6cdced0adb9808a27787b8cc5d58459170840
NODE_NAME=master
gitlabSourceRepoSshUrl=ssh://git@localhost:10022/root/anton.git
gitlabTargetRepoName=anton
gitlabTargetNamespace=root
gitlabMergeRequestLabels=deploy,bug
STAGE_NAME=Prepare
CA_CERTIFICATES_JAVA_VERSION=20170531+nmu1
gitlabSourceRepoHomepage=http://localhost:10080/root/anton
EXECUTOR_NUMBER=0
gitlabBranch=test
BUILD_DISPLAY_NAME=#5
gitlabSourceBranch=test
gitlabUserEmail=admin@example.com
JENKINS_VERSION=2.89.4
HUDSON_HOME=/var/jenkins_home
JAVA_DEBIAN_VERSION=8u151-b12-1~deb9u1
JOB_BASE_NAME=anton
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
BUILD_ID=5
BUILD_TAG=jenkins-anton-5
gitlabMergeRequestId=1
gitlabActionType=MERGE
JENKINS_URL=http://localhost:8080/
LANG=C.UTF-8
JOB_URL=http://localhost:8080/job/anton/
gitlabSourceRepoName=anton
gitlabSourceNamespace=root
BUILD_NUMBER=5
JENKINS_NODE_COOKIE=63f85221-303e-4906-aca5-59985eb83c63
RUN_DISPLAY_URL=http://localhost:8080/job/anton/5/display/redirect
JENKINS_SLAVE_AGENT_PORT=50000
HUDSON_SERVER_COOKIE=e3dff077c89c3538
JOB_DISPLAY_URL=http://localhost:8080/job/anton/display/redirect
JOB_NAME=anton
COPY_REFERENCE_FILE_LOG=/var/jenkins_home/copy_reference_file.log
PWD=/var/jenkins_home/workspace/anton
JAVA_HOME=/docker-java-home
gitlabUserName=Administrator

Limitations:

• Does not work when commenting on the MR, because that GitLab webhook does not include merge request labels.