add ability to select system test patch-sets for verification

Change-Id: Ida4f80d4287481c45ce636110c1eec17854e18c4
diff --git a/jjb/pipeline/voltha-dt-physical-functional-tests.groovy b/jjb/pipeline/voltha-dt-physical-functional-tests.groovy
index 1ebd19d..f07a112 100644
--- a/jjb/pipeline/voltha-dt-physical-functional-tests.groovy
+++ b/jjb/pipeline/voltha-dt-physical-functional-tests.groovy
@@ -66,6 +66,14 @@
             [$class: 'CloneOption', depth: 0, noTags: false, reference: '', shallow: false],
           ],
         ])
+        script {
+          sh(script:"""
+            if [ '${volthaSystemTestsChange}' != '' ] ; then
+              cd $WORKSPACE/voltha-system-tests;
+              git fetch https://gerrit.opencord.org/voltha-system-tests ${volthaSystemTestsChange} && git checkout FETCH_HEAD
+            fi
+            """)
+        }
       }
     }
     stage('Clone cord-tester') {