Replace all Ubuntu 16.04 with Ubuntu18.04 image

Change-Id: I86b84957145f7c1f253d3158e42e015c322788eb
diff --git a/jjb/xos-unit.yaml b/jjb/xos-unit.yaml
index 89fd6c1..12190b5 100644
--- a/jjb/xos-unit.yaml
+++ b/jjb/xos-unit.yaml
@@ -32,7 +32,7 @@
           branch: 'master'
           destination-dir: 'cord'
 
-    node: 'ubuntu16.04-basebuild-1c-2g'
+    node: 'ubuntu18.04-basebuild-1c-2g'
     project-type: freestyle
     concurrent: true