Replace all Ubuntu 16.04 with Ubuntu18.04 image

Change-Id: I86b84957145f7c1f253d3158e42e015c322788eb
diff --git a/jjb/charts.yaml b/jjb/charts.yaml
index 9908e7c..9999d4b 100644
--- a/jjb/charts.yaml
+++ b/jjb/charts.yaml
@@ -45,7 +45,7 @@
           choosing-strategy: 'gerrit'
           jenkins-ssh-credential: '{jenkins-ssh-credential}'
 
-    node: 'ubuntu16.04-basebuild-2c-4g'
+    node: 'ubuntu18.04-basebuild-2c-4g'
     project-type: freestyle
     concurrent: false
 
@@ -153,7 +153,7 @@
           choosing-strategy: gerrit
           jenkins-ssh-credential: '{jenkins-ssh-credential}'
 
-    node: 'ubuntu16.04-basebuild-2c-4g'
+    node: 'ubuntu18.04-basebuild-2c-4g'
     project-type: freestyle
     concurrent: true