Fix failure during Monasca Grafana upgrade

The task "Stopping all Monasca Grafana instances but the first node"
can fail with:

    error while evaluating conditional (monasca_grafana_differs['result']): 'dict object' has no attribute 'result'

This is fixed by running this task on the same set of hosts than the
task defining monasca_grafana_differs, i.e. groups['monasca-grafana'].

Change-Id: I6ad0256fb2a3cdc91dddf441e5e1c41f4ac69017
Closes-Bug: #1907689

ansible/roles/monasca/tasks/upgrade.yml

@@ -29,6 +29,7 @@
     common_options: "{{ docker_common_options }}"
     name: "{{ service.container_name }}"
   when:
+    - inventory_hostname in groups['monasca-grafana']
     - inventory_hostname != groups['monasca-grafana']|first
     - monasca_grafana_differs['result']