[SDFAB-189] Update ONOS CI and test new image

This is the part 2 (other changes will follow).
Use new ubuntu 20.04 based executor for snapshot jobs

Change-Id: Ie38b56d6e1c97e3cbe57f269e6773f2b7b5a3274
diff --git a/jjb/onos/onos-verify.yaml b/jjb/onos/onos-verify.yaml
index d524e89..e517f48 100644
--- a/jjb/onos/onos-verify.yaml
+++ b/jjb/onos/onos-verify.yaml
@@ -900,7 +900,7 @@
       project: onos
 
       build-timeout: '240'
-      build-node: 'ubuntu16.04-basebuild-8c-15g'
+      build-node: 'ubuntu20.04-basebuild-8c-15g'
 
       stream:
           - 'master'