Replace all Ubuntu 16.04 with Ubuntu18.04 image

Change-Id: I86b84957145f7c1f253d3158e42e015c322788eb
diff --git a/jjb/verify/device-management.yaml b/jjb/verify/device-management.yaml
index 58f2dec..0179a40 100644
--- a/jjb/verify/device-management.yaml
+++ b/jjb/verify/device-management.yaml
@@ -19,7 +19,7 @@
           dependency-jobs: 'verify_device-management_licensed'
       - 'make-unit-test':
           junit-allow-empty-results: true
-          build-node: 'ubuntu16.04-basebuild-2c-4g'
+          build-node: 'ubuntu18.04-basebuild-2c-4g'
       - 'device-management-patch-test':
           pipeline-script: 'voltha/master/device-management-mock-tests.groovy'