Heat stack-update fails on first try with "Port (...) is still in use" and then succeeds later

Issue

When running a heat stack-update on a template which contains a server of type OS::Nova::Server which uses a port of type OS::Neutron::Port, heat will fail with the following error message as seen in heat event-list <stack name>

[stack@undercloud heat]$ heat event-list cep-dev-app | grep CREATE_FAILED
| cep_app_instance  | b5ff7de9-34b1-4c03-bc31-a9053b928adc | Conflict: resources.cep_app_instance: Port ee59493b-ac40-4773-a22d-e5e83ac515a6 is still in use. (HTTP 409) (Request-ID: req-478faa90-9198-40e0-9f25-519ce6f75504) | CREATE_FAILED      | 2016-05-31T00:32:59Z |

A subsequent heat stack-update will succeed.