Replace all Ubuntu 16.04 with Ubuntu18.04 image

Change-Id: I86b84957145f7c1f253d3158e42e015c322788eb
diff --git a/jjb/verify/openolt-test.yaml b/jjb/verify/openolt-test.yaml
index c8ef753..67d0f99 100644
--- a/jjb/verify/openolt-test.yaml
+++ b/jjb/verify/openolt-test.yaml
@@ -16,7 +16,7 @@
       - 'verify-licensed'
       - 'tag-collision-reject'
       - 'make-unit-test':
-          build-node: 'ubuntu16.04-basebuild-2c-4g'
+          build-node: 'ubuntu18.04-basebuild-2c-4g'
           build-timeout: 60
           unit-test-targets: 'lint docker-build'
           junit-allow-empty-results: true