Add a TODO note for resource classes constants
Add a TODO note for resource classes constants

The os-resource-classes lib is the official source of the standard
placement resource classes. Currently, the neutron-lib contains a copy
of those constants. This code duplication shall be fixed with next
major neutron-lib release. For the time being, add a TODO note
informing that those constants shouldn't be used.

Related-Bug: #1934256
Change-Id: Iee08835fd30d43249a261aa0c0a0b6f2cda08ea4

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