Fix Cinder multinode grenade job

JJB treats lists differently than single values in `node` properties.
This was causing no worker being registered for the Cinder multinode
grenade jobs. This commit switches from list notation in `node`
properties for gate-grenade-dsvm-cinder-multinode-* jobs.

Change-Id: I0382470bb666cbf7ed64e1427259dd96f1b06b5e

jenkins/jobs/projects.yaml

@@ -1228,18 +1228,14 @@
             - ubuntu-xenial
       - '{pipeline}-grenade-dsvm-cinder-multinode-{node-release}{job-suffix}':
           pipeline: gate
-          node:
-            - ubuntu-trusty-2-node
-          node-release:
-            - ubuntu-trusty
+          node: ubuntu-trusty-2-node
+          node-release: ubuntu-trusty
           job-suffix: ''
           branch-override: default
       - '{pipeline}-grenade-dsvm-cinder-multinode-{node-release}{job-suffix}':
           pipeline: gate
-          node:
-            - ubuntu-xenial-2-node
-          node-release:
-            - ubuntu-xenial
+          node: ubuntu-xenial-2-node
+          node-release: ubuntu-xenial
           job-suffix: ''
           branch-override: default
       - api-ref-jobs: