Replace all Ubuntu 16.04 with Ubuntu18.04 image

Change-Id: I86b84957145f7c1f253d3158e42e015c322788eb
diff --git a/jjb/verify/openolt.yaml b/jjb/verify/openolt.yaml
index ef8ab38..04ae7b4 100644
--- a/jjb/verify/openolt.yaml
+++ b/jjb/verify/openolt.yaml
@@ -15,7 +15,7 @@
       - 'verify-licensed'
       - 'tag-collision-reject'
       - 'make-unit-test':
-          build-node: 'ubuntu16.04-basebuild-2c-4g'
+          build-node: 'ubuntu18.04-basebuild-2c-4g'
           unit-test-targets: 'test'
           # openolt uses gtest which outputs in xunit format
           junit-allow-empty-results: true