Need a method for doing an oc tag based promotion

[etsauer]

@redhat-cop/developer-workflow

[syvanen]

There's actually not much magic to this:

    stage("Promote image") {
      steps {
        script {
          openshift.withCluster() {
            openshift.tag("${STAGE1}/${APP_NAME}:latest", "${STAGE2}/${APP_NAME}:latest")
          }
        }
      }
    }

[pabrahamsson]

Fixed by #16