Replace all Ubuntu 16.04 with Ubuntu18.04 image

Change-Id: I86b84957145f7c1f253d3158e42e015c322788eb
diff --git a/jjb/verify/voltha-go.yaml b/jjb/verify/voltha-go.yaml
index d3db340..26eaaf8 100644
--- a/jjb/verify/voltha-go.yaml
+++ b/jjb/verify/voltha-go.yaml
@@ -24,7 +24,7 @@
       - 'tag-collision-reject':
           dependency-jobs: 'verify_voltha-go_licensed'
       - 'make-unit-test':
-          build-node: 'ubuntu16.04-basebuild-4c-8g'
+          build-node: 'ubuntu18.04-basebuild-4c-8g'
           build-timeout: 20
           dest-gopath: "github.com/opencord"
           name-extension: "-lint"
@@ -32,7 +32,7 @@
           unit-test-keep-going: 'true'
           junit-allow-empty-results: true
       - 'make-unit-test':
-          build-node: 'ubuntu16.04-basebuild-2c-4g'
+          build-node: 'ubuntu18.04-basebuild-2c-4g'
           dest-gopath: "github.com/opencord"
           name-extension: "-tests"
           unit-test-targets: 'test'