Neutron is not creating DNS records when recreating an instance with the same floating IP

When creating an instance for the first time and assigning a floating IP to it. A DNS record is created. When removing the instance using the API and recreating it. The DNS record gets deleted but is not created again.

The issue is present for us on the Stein release (14.4.2).

To reproduce (using the CLI):
* Create project
* Create an internal network and subnet.
* Create router and add an External gateway.
* Create DNS zone (we also use Bind9 as an external DNS provider).
* Create an instance on the internal network
* Create a floating IP and assign it to an instance. A DNS record is created.
* Delete the instance. Its DNS record is *not* removed.
* Recreate the instance with the same steps. The DNS record is removed.
* Recreating the instance again will not create a DNS record.

This causes some issues when recreating an instance with the same floating IP.
Because the DNS record does not get created when recreating the instance, we can't connect to the instance using the hostname.

Some other things we noticed were

When removing floating IPs from an instance using the CLI (OpenStack server remove floating ip <servername><IP>) the DNS entry in the zone is not deleted.
When you use the Horizon dashboard to remove the floating IP the DNS record is removed.

In the neutron database. The table "floatingipdnses" is updated when a floating IP is assigned to an instance for the first time.
When removing the floating IP with the CLI, the record in this table is not removed.
When removing the floating IP with the Horizon dashboard, the record in this table is removed.

If we create a floating IP with dns_domain and dns_name configured, It will create a DNS record immediately (without an instance).
if we assign that floating IP to an instance and delete the instance. The DNS record gets removed and is not created again when using that floating IP.