Migrate to ARM Ubuntu18.04 node

Ubuntu 16.04 still has SSH and Kernel issues.
Migrating to Ubuntu18.04 since we have a good
instance now.

TODO: Disable Ubuntu16.04 arm nodes.
- Currently Ubuntu18.04 arm nodes are under more test.
- No additional costs since no job is using 16.04 arm
  nodes.
- Once confirmed, these will be disabled.

Change-Id: I1ef6dfc04aa6e85531f824e24ad517137a7080ca
Issue-ID: CIMAN-33
Signed-off-by: Jessica Wagantall <jwagantall@linuxfoundation.org>
diff --git a/jjb/msb/msb-discovery.yaml b/jjb/msb/msb-discovery.yaml
index b38802b..cbe5505 100644
--- a/jjb/msb/msb-discovery.yaml
+++ b/jjb/msb/msb-discovery.yaml
@@ -36,7 +36,7 @@
 #          mvn-goals: 'install'
 #          script:
 #            !include-raw-escape: 'build-discovery-docker-image.sh'
-#          build-node: ubuntu1604-docker-arm64-4c-2g
+#          build-node: ubuntu1804-docker-arm64-4c-2g
 #    project: 'msb/discovery'
 #    stream:
 #      - 'master':