Replace all Ubuntu 16.04 with Ubuntu18.04 image

Change-Id: I86b84957145f7c1f253d3158e42e015c322788eb
diff --git a/jjb/gui-unit.yaml b/jjb/gui-unit.yaml
index 6cb5c2e..bb10618 100644
--- a/jjb/gui-unit.yaml
+++ b/jjb/gui-unit.yaml
@@ -40,7 +40,7 @@
           jenkins-ssh-credential: '{jenkins-ssh-credential}'
 
     # `npm install` fails on 1Gb RAM
-    node: 'ubuntu16.04-basebuild-1c-2g'
+    node: 'ubuntu18.04-basebuild-1c-2g'
     project-type: freestyle
     concurrent: true