fix(ceilometer): process instance.exists msgs

By default, ceilometer creates the event but does not update the gnocchi
resource with the audit times. Let's do that so that we have some
insurance on the fact that the gnocchi resource is up-to-date.

base-helm-configs/ceilometer/ceilometer-helm-overrides.yaml

@@ -889,6 +889,11 @@ conf:
           server_group: resource_metadata.user_metadata.server_group
         event_delete: compute.instance.delete.start
         event_create: compute.instance.create.end
+        event_update:
+          - compute.instance.update
+          - compute.instance.exists
+          - compute.instance.resize_confirm.end
+          - compute.instance.resize_revert.end
         event_attributes:
           id: instance_id
           display_name: display_name