Replace all Ubuntu 16.04 with Ubuntu18.04 image

Change-Id: I86b84957145f7c1f253d3158e42e015c322788eb
diff --git a/jjb/verify/abstract-olt.yaml b/jjb/verify/abstract-olt.yaml
index 3aab818..fcd9bff 100644
--- a/jjb/verify/abstract-olt.yaml
+++ b/jjb/verify/abstract-olt.yaml
@@ -51,7 +51,7 @@
           jenkins-ssh-credential: '{jenkins-ssh-credential}'
           basedir: '{project}'
 
-    node: 'ubuntu16.04-basebuild-1c-2g'
+    node: 'ubuntu18.04-basebuild-1c-2g'
     project-type: freestyle
     concurrent: true