Temporary change to pull master branch for voltha-system-tests for voltha-2.7 jobs

Change-Id: I008f774af8ff639c15ce0f3bbd2c4d0e9dbe5d69
diff --git a/jjb/pipeline/voltha-physical-functional-tests.groovy b/jjb/pipeline/voltha-physical-functional-tests.groovy
index a2d9c8d..74f8c53 100644
--- a/jjb/pipeline/voltha-physical-functional-tests.groovy
+++ b/jjb/pipeline/voltha-physical-functional-tests.groovy
@@ -58,7 +58,7 @@
             url: "https://gerrit.opencord.org/voltha-system-tests",
             refspec: "${volthaSystemTestsChange}"
           ]],
-          branches: [[ name: "${branch}", ]],
+          branches: [[ name: "master", ]],
           extensions: [
             [$class: 'WipeWorkspace'],
             [$class: 'RelativeTargetDirectory', relativeTargetDir: "voltha-system-tests"],