Updating helm-repo to get latest chart before installing pod in voltha-scale-measurement job

Change-Id: Ie6b5efd9461af68240b6af8412bc12fc3c8713e8
diff --git a/jjb/pipeline/voltha-scale-measurements.groovy b/jjb/pipeline/voltha-scale-measurements.groovy
index b3cdef7..4fcb401 100644
--- a/jjb/pipeline/voltha-scale-measurements.groovy
+++ b/jjb/pipeline/voltha-scale-measurements.groovy
@@ -56,6 +56,7 @@
       }
       steps {
         sh '''
+          helm repo update
           helm install -n nem-monitoring cord/nem-monitoring
 
           IFS=: read -r onosRepo onosTag <<< ${onosImg}