Fix no-replace-server to accurately preview update

This override of OS::Nova::Server needs to reflect the fact
that it never replaces on update or the update --dry-run output
ends up being wrong.

Closes-Bug: 1561076
Change-Id: I9256872b877fbe7f91befb52995c62de006210ef

undercloud_heat_plugins/server_update_allowed.py

@@ -21,6 +21,10 @@ class ServerUpdateAllowed(server.Server):
 
     update_allowed_properties = server.Server.properties_schema.keys()
 
+    def needs_replace_with_prop_diff(self, changed_properties_set,
+                                     after_props, before_props):
+        return False
+
 
 def resource_mapping():
     return {'OS::Nova::Server': ServerUpdateAllowed}