Replace all Ubuntu 16.04 with Ubuntu18.04 image

Change-Id: I86b84957145f7c1f253d3158e42e015c322788eb
diff --git a/jjb/verify/up4.yaml b/jjb/verify/up4.yaml
index 17788a6..dbb83bd 100644
--- a/jjb/verify/up4.yaml
+++ b/jjb/verify/up4.yaml
@@ -13,7 +13,7 @@
           stream: "master"
           # As we run integration tests with 3 ONOS instances,
           # we need a beefy node.
-          build-node: "ubuntu16.04-basebuild-8c-15g"
+          build-node: "ubuntu18.04-basebuild-8c-15g"
 
 - job-group:
     name: "up4-jobs"