Replace all Ubuntu 16.04 with Ubuntu18.04 image

Change-Id: I86b84957145f7c1f253d3158e42e015c322788eb
diff --git a/jjb/verify/xos.yaml b/jjb/verify/xos.yaml
index ff385f5..b33d7d2 100644
--- a/jjb/verify/xos.yaml
+++ b/jjb/verify/xos.yaml
@@ -31,7 +31,7 @@
     jobs:
       - 'python-unit-test':
           build-timeout: 45
-          build-node: 'ubuntu16.04-basebuild-2c-4g'
+          build-node: 'ubuntu18.04-basebuild-2c-4g'
       - 'api-test':
           pipeline-script: 'xos-core.groovy'