Replace all Ubuntu 16.04 with Ubuntu18.04 image

Change-Id: I86b84957145f7c1f253d3158e42e015c322788eb
diff --git a/jjb/verify/voltha-bbsim.yaml b/jjb/verify/voltha-bbsim.yaml
index 1ad61e9..5cdf7ad 100644
--- a/jjb/verify/voltha-bbsim.yaml
+++ b/jjb/verify/voltha-bbsim.yaml
@@ -21,7 +21,7 @@
           unit-test-targets: 'test'
           dest-gopath: "github.com/opencord"
           junit-allow-empty-results: true
-          build-node: 'ubuntu16.04-basebuild-1c-2g'
+          build-node: 'ubuntu18.04-basebuild-1c-2g'
 
 - job-group:
     name: 'publish-voltha-bbsim-jobs'