Revert "Switch to RHEL8-Cluster-Image-Builder in RAN-Container-Parent"

This reverts commit 5008eb4e38.

The RHEL8-Cluster-Image-Builder has problems and is not ready. This
commit reverts the switch, and uses RHEL8-Image-Builder instead.

ci-scripts/Jenkinsfile-GitLab-Container

@@ -136,16 +136,16 @@ pipeline {
             }
           }
         }
-        stage ("RHEL8-Cluster-Image-Builder") {
+        stage ("RHEL8 Build") {
           steps {
             script {
-              triggerSlaveJob ('RAN-RHEL8-Cluster-Image-Builder', 'RHEL8-Cluster-Image-Builder')
+              triggerSlaveJob ('RAN-RHEL8-Image-Builder', 'RHEL8-Images-Build')
             }
           }
           post {
             always {
               script {
-                finalizeSlaveJob('RAN-RHEL8-Cluster-Image-Builder')
+                finalizeSlaveJob('RAN-RHEL8-Image-Builder')
               }
             }
             failure {