Move salt formulas test to xenial jenkins node

Currently some tests are not build-able on jenkins bionic nodes

Related-bug: PROD-36062
Change-Id: I99990428ae976f2e8ee9d9d091c1a5fb7f0796c4
diff --git a/test-salt-models-pipeline.groovy b/test-salt-models-pipeline.groovy
index 3b88aee..0b4a4d6 100644
--- a/test-salt-models-pipeline.groovy
+++ b/test-salt-models-pipeline.groovy
@@ -86,8 +86,10 @@
   return true;
 }
 
+def nodeLabel = 'old16.04'
+
 timeout(time: 12, unit: 'HOURS') {
-  node("python") {
+  node(nodeLabel) {
     try{
       stage("checkout") {
         if (gerritRef) {