Do not pull images in the experimental multi-stack job (avoid docker limit)

Change-Id: I6fc1953c6e291fc84f31f0bf0894fe63352ce9d7
diff --git a/jjb/voltha-scale.yaml b/jjb/voltha-scale.yaml
index 99f2813..ba193c1 100644
--- a/jjb/voltha-scale.yaml
+++ b/jjb/voltha-scale.yaml
@@ -101,7 +101,7 @@
           withEapol: false
           withDhcp: false
           withIgmp: false
-          extraHelmFlags: '--set authRetry=true,dhcpRetry=true'
+          extraHelmFlags: '--set authRetry=true,dhcpRetry=true --set defaults.image_pullPolicy=IfNotPresent'
           volthaSystemTestsChange: refs/changes/27/21627/4
           kindVolthaChange: refs/changes/85/21485/13