Add ServerDeletionPolicy parameter

With this we could potentially delete or update the heat stack without
deleting any servers, e.g if we wanted to switch from heat/nova managed
server resources to some which are externally managed via Ironic directly.

Partially-Implements: blueprint nova-less-deploy
Change-Id: I2b871c1f096013956904441cddc36cb2404e477a

puppet/role.role.j2.yaml

@@ -245,6 +245,13 @@ parameters:
     default: 15
     type: number
 
+  ServerDeletionPolicy:
+    description: Whether to retain or delete servers on deletion of the stack
+    type: string
+    default: delete
+    constraints:
+      - allowed_values: [delete, retain]
+
 {% if role.uses_deprecated_params|default(false) %}
 parameter_groups:
 - label: deprecated
@@ -305,6 +312,7 @@ conditions:
 resources:
   {{server_resource_name}}:
     type: OS::TripleO::{{role.name}}Server
+    deletion_policy: {get_param: ServerDeletionPolicy}
     metadata:
       os-collect-config:
         command: {get_param: ConfigCommand}