Replace all Ubuntu 16.04 with Ubuntu18.04 image

Change-Id: I86b84957145f7c1f253d3158e42e015c322788eb
diff --git a/jjb/verify/ponsim.yaml b/jjb/verify/ponsim.yaml
index 68cf602..527c0cd 100644
--- a/jjb/verify/ponsim.yaml
+++ b/jjb/verify/ponsim.yaml
@@ -18,7 +18,7 @@
       - 'tag-collision-reject':
           dependency-jobs: 'verify_ponsim_licensed'
       - 'make-unit-test':
-          build-node: 'ubuntu16.04-basebuild-1c-2g'
+          build-node: 'ubuntu18.04-basebuild-1c-2g'
           dest-gopath: "github.com/opencord"
           unit-test-targets: 'test docker-build'
           unit-test-keep-going: 'true'