Moving to 18.04 for voltha-system-tests

Change-Id: Ide489e73e9487c5702ab228916f19571593ef798
diff --git a/jjb/verify/voltha-system-tests.yaml b/jjb/verify/voltha-system-tests.yaml
index f7efbab..3624903 100644
--- a/jjb/verify/voltha-system-tests.yaml
+++ b/jjb/verify/voltha-system-tests.yaml
@@ -7,12 +7,15 @@
 
     jobs:
       - 'verify-voltha-system-tests-jobs':
+          buildNode: 'ubuntu18.04-basebuild-4c-8g'
           branch-regexp: '{all-branches-regexp}'
       - 'verify-voltha-system-tests-jobs-voltha-2.7':
+          buildNode:  'ubuntu18.04-basebuild-4c-8g'
           name-extension: '-voltha-2.7'
           override-branch: 'voltha-2.7'
           branch-regexp: '{kind-voltha-regexp}'
       - 'verify-voltha-system-tests-jobs-master':
+          buildNode:  'ubuntu18.04-basebuild-4c-8g'
           branch-regexp: '^master$'
 
 - job-group: