Unsetting VOLTHA_API_PORT between stack deployment otherwise they'll be forwarded on the same port

Change-Id: Ibbdf8619182bf672b5661ee715c0d8db55803b12
diff --git a/jjb/pipeline/voltha-scale-multi-stack.groovy b/jjb/pipeline/voltha-scale-multi-stack.groovy
index a7d3f71..6f053c7 100644
--- a/jjb/pipeline/voltha-scale-multi-stack.groovy
+++ b/jjb/pipeline/voltha-scale-multi-stack.groovy
@@ -555,6 +555,10 @@
   for (int i = 1; i <= numberOfStacks.toInteger(); i++) {
     stage("Deploy VOLTHA stack " + i) {
       sh returnStdout: false, script: """
+
+      # unset voltha-api port so that the port is forwarded on a new one
+      unset VOLTHA_API_PORT
+
       cd $WORKSPACE/kind-voltha/
 
       export NAME=voltha${i}