Add DeployIdentifier to Nova discover hosts container

Resolves an issue during scale out where the Nova init
container that runs discover hosts wasn't executing on a Heat
stack update if the container name and configs all stayed the same.

Change-Id: Ie2ecd3dbddb1cf3ee5bba6f7b33e11bc9b6b8b4e
Closes-bug: 1733966

docker/services/nova-api.yaml

@@ -43,6 +43,12 @@ parameters:
     default: false
     description: Remove package if the service is being disabled during upgrade
     type: boolean
+  DeployIdentifier:
+    default: ''
+    type: string
+    description: >
+      Setting this to a unique value will re-run any deployment tasks which
+      perform configuration on a Heat stack-update.
 
 conditions:
 
@@ -289,6 +295,13 @@ outputs:
                   - /var/lib/docker-config-scripts/nova_api_discover_hosts.sh:/nova_api_discover_hosts.sh:ro
             user: root
             command: "/usr/bin/bootstrap_host_exec nova_api /nova_api_discover_hosts.sh"
+            environment:
+              # NOTE: this should force this container to re-run on each
+              # update (scale-out, etc.)
+              - list_join:
+                  - ''
+                  - - 'TRIPLEO_DEPLOY_IDENTIFIER='
+                    - {get_param: DeployIdentifier}
       metadata_settings:
         get_attr: [NovaApiBase, role_data, metadata_settings]
       host_prep_tasks: {get_attr: [NovaApiLogging, host_prep_tasks]}