move nvp-qos extension to constants file

[Aaron Rosen]

move nvp-qos extension to constants file to be consistent with other extensions.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/99485

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/99485
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=41bb7dc7b9886a8000c3c1759b2c305ec91824fd
Submitter: Jenkins
Branch: master

commit 41bb7dc7b9886a8000c3c1759b2c305ec91824fd
Author: Aaron Rosen <email address hidden>
Date: Wed Jun 11 14:25:29 2014 -0700

    Migrate nvp-qos to generic name qos-queue

    In Juno, the nvp-qos extension was renamed to qos-queue in favor of having
    a generic name. In order to maintain backwards compatibility neutron
    advertises both nvp-qos and qos-queue (since icehouse). This patch phases out
    nvp-qos from nova so that it uses qos-queue so that in Kxxx we'll be able to
    drop nvp-qos from neutron completely.

    In addition this patch moves qos-queue to a constant like the other extensions
    and improves a doc string.

    Change-Id: Id43ea931c995865ddc3455abfb625d86dd93e78f
    Closes-bug: 1329075