Introduce a constants file for later use for symbols in Placement,
including constants for:
* custom trait prefixes
* resource class names (though that should be replaced as soon as we
have an os-resource-classes package)
* known inventory parameters to validate config options
Change-Id: I3a5afd04ce8b5919efdb7591e3717742dd27151e
Partial-Bug: #1578989
See-Also: https://review.openstack.org/502306 (nova spec)
See-Also: https://review.openstack.org/508149 (neutron spec)