Draft: Repair cluster-build pipeline

The introduction of building RHEL8 images on the cluster in MR !1681 (merged) had a flaw: it would recreate image streams on every CI run, thereby losing the association to images in the image registry. In the CI, we change from time to time the ran-base image between tags develop and ci-temp. By recreating the image streams, we quickly lost the association of ran-base:develop, which the CI assumed to "just be there". In this case, most builds would fail, basically bringing the CI to a halt. The actually important change here is in 83736340

Marking as BUILD-ONLY as we cannot test this in the CI: the CI will trigger the normal RHEL8 build on asterix. I will reproduce the scenario we had previously, i.e., trigger this pipeline multiple times in a way that builds will change the ran-base image between ran-base:develop and ran-base:ci-temp. The expected outcome is to always have a successful build, of course.

@lthomas I have additional changes to not use the SSHConnection class, but this will come later. The same with the cleanup of the physims.

ci-scripts/Jenkinsfile-GitLab-Container

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