Add list of all working DSCP marks
Add list of all working DSCP marks

There is no place in the documentation that explicitly lists the valid
DSCP marks, except for an incomplete hint in the DSCP spec in
neutron-specs.  This provides an explicit list.

Change-Id: Ic350c88e59c33d98b54086707c9add05cf137dc2
Closes-Bug: #1781915

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