Use ExtensionDescriptor from neutron-lib
Use ExtensionDescriptor from neutron-lib

ExtensionDescriptor has been rehomed to neutron-lib and is being
removed from neutron core.

See https://review.openstack.org/398113

Depends-On: I5a111e0033e518e39b3042f047ee9eebba77a0d5

Change-Id: I14d55a5b1b419eb39794f5f35bd219d6e83f48f4

openstack/networking-bgpvpn,openstack/networking-bgpvpn,openstack/networking-bgpvpn