Replace all Ubuntu 16.04 with Ubuntu18.04 image

Change-Id: I86b84957145f7c1f253d3158e42e015c322788eb
diff --git a/jjb/voltha-publish.yaml b/jjb/voltha-publish.yaml
index d8b8246..aa68028 100644
--- a/jjb/voltha-publish.yaml
+++ b/jjb/voltha-publish.yaml
@@ -9,7 +9,7 @@
       - 'voltha-publish':
           project-regexp: 'voltha'
           branch-regexp: '^(master|voltha-1.6|voltha-1.7)$'
-          build-node: 'ubuntu16.04-basebuild-1c-2g'
+          build-node: 'ubuntu18.04-basebuild-1c-2g'
           dependency-jobs: ''
 
 - job-template: