Replace all Ubuntu 16.04 with Ubuntu18.04 image

Change-Id: I86b84957145f7c1f253d3158e42e015c322788eb
diff --git a/jjb/bbr.yaml b/jjb/bbr.yaml
index f5bd5c9..14b4caa 100644
--- a/jjb/bbr.yaml
+++ b/jjb/bbr.yaml
@@ -48,5 +48,5 @@
     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'