[api-ref] Add new 409 response entry of floating IP creation error
[api-ref] Add new 409 response entry of floating IP creation error

When floating IP is created and the recordset quota value of the
external dns service (e.g. designate) exceeds limit, the recordset
creation fails. This throws ExternalDNSOverQuota 409 error.

Related-Bug: #1923700
Change-Id: I7d4324260318a75b5e3bcf95e5f697536d020b34

openstack/neutron-lib,openstack/neutron-lib,openstack/neutron-lib,openstack/neutron-lib