Retrieve the DSCP valid marks from the API

Instead of relying on a constant, read the DSCP valid supported marks
from the API, using the 'qos-rule-type-details' call [1].

[1]https://docs.openstack.org/api-ref/network/v2/index.html?expanded=show-qos-rule-type-details-detail#show-qos-rule-type-details

Related-Bug: #1987378
Change-Id: I207e32a966fb43d6f34fac4a160d69944a19217b
1 file changed