Fixed WsCleanup step in voltha physical pod jobs

Change-Id: Ifd3bf0e408ab2a183374fab81812734b45fef4d6
diff --git a/jjb/pipeline/voltha/voltha-2.8/voltha-physical-soak-dt-tests.groovy b/jjb/pipeline/voltha/voltha-2.8/voltha-physical-soak-dt-tests.groovy
index 117fc4c..8a29330 100644
--- a/jjb/pipeline/voltha/voltha-2.8/voltha-physical-soak-dt-tests.groovy
+++ b/jjb/pipeline/voltha/voltha-2.8/voltha-physical-soak-dt-tests.groovy
@@ -36,6 +36,7 @@
   stages {
     stage('Clone voltha-system-tests') {
       steps {
+        step([$class: 'WsCleanup'])
         checkout([
           $class: 'GitSCM',
           userRemoteConfigs: [[