Metadata addresses
Metadata addresses

Define well-known metadata addresses in neutron-lib so they are easily
shared between various implementations like dhcp-agent and ovn.

Change-Id: Ic382feb7fca036b3c7aae5f21399b7639520a92b
Partial-Bug: #1460177
Related-Change: https://review.opendev.org/738247

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