Fixed Cleanup section in Jenkins-voltha-build

Change-Id: Iae1e859745c8aa8f9314c091ac0428b9f579e7f0
diff --git a/Jenkinsfile-voltha-build b/Jenkinsfile-voltha-build
index cb2f59f..4dc9ee5 100644
--- a/Jenkinsfile-voltha-build
+++ b/Jenkinsfile-voltha-build
@@ -48,7 +48,7 @@
                     NAMESPACES="voltha infra default"
                     for NS in \$NAMESPACES
                     do
-                        for hchart in \$(helm list -n $NS -q | grep -E -v 'docker-registry|kafkacat');
+                        for hchart in \$(helm list -n \$NS -q | grep -E -v 'docker-registry|kafkacat');
                         do
                             echo "Purging chart: \${hchart}"
                             helm delete -n \$NS "\${hchart}"