use dns domain ports api def from neutron-lib
use dns domain ports api def from neutron-lib

The dns domain ports extension's API definition was rehomed into
neutron-lib with I79a96e72daa5baf579fcece2a703a47b8059423e
This patch consumes it by removing the rehomed code and using the
APIExtensionDescriptor for the extensions parent class.

NeutronLibImpact

Change-Id: I8517a5b7c1ae7f4909f4c9829cf8ddea865f5210

mahak/neutron,openstack/neutron,mahak/neutron,noironetworks/neutron,eayunstack/neutron,mahak/neutron,noironetworks/neutron,huntxu/neutron,openstack/neutron,openstack/neutron,eayunstack/neutron,huntxu/neutron