Replace all Ubuntu 16.04 with Ubuntu18.04 image

Change-Id: I86b84957145f7c1f253d3158e42e015c322788eb
diff --git a/jjb/xos-upgrade.yaml b/jjb/xos-upgrade.yaml
index 506d5dd..4889246 100644
--- a/jjb/xos-upgrade.yaml
+++ b/jjb/xos-upgrade.yaml
@@ -29,7 +29,7 @@
     parameters:
       - string:
           name: buildNode
-          default: 'ubuntu16.04-basebuild-4c-8g'
+          default: 'ubuntu18.04-basebuild-4c-8g'
           description: 'Name of the Jenkins node to run the job on'
 
       - string: