Replace all Ubuntu 16.04 with Ubuntu18.04 image

Change-Id: I86b84957145f7c1f253d3158e42e015c322788eb
diff --git a/jjb/docker-publish.yaml b/jjb/docker-publish.yaml
index d264eb9..dc0185c 100644
--- a/jjb/docker-publish.yaml
+++ b/jjb/docker-publish.yaml
@@ -28,7 +28,7 @@
     parameters:
       - string:
           name: buildNode
-          default: 'ubuntu16.04-basebuild-1c-2g'
+          default: 'ubuntu18.04-basebuild-1c-2g'
           description: 'Name of the Jenkins build executor to run the job on'
 
       - string: