Replace all Ubuntu 16.04 with Ubuntu18.04 image

Change-Id: I86b84957145f7c1f253d3158e42e015c322788eb
diff --git a/jjb/voltha-unit-test.yaml b/jjb/voltha-unit-test.yaml
index c2132fa..f4225a3 100644
--- a/jjb/voltha-unit-test.yaml
+++ b/jjb/voltha-unit-test.yaml
@@ -37,7 +37,7 @@
           choosing-strategy: gerrit
           jenkins-ssh-credential: '{jenkins-ssh-credential}'
 
-    node: 'ubuntu16.04-basebuild-4c-8g'
+    node: 'ubuntu18.04-basebuild-4c-8g'
     project-type: freestyle
     concurrent: true