Quota model does not use HasTenant mixin

[Salvatore Orlando]

This is a minor issue, but in the future (ie: when we'll finally move to renaming tenant_id to project_id) this could cause problem.
As it's a super easy fix it might be worth doing it.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/174570

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/174570
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=decdf03c61f303fcfc82fe601beb4096d3305536
Submitter: Jenkins
Branch: master

commit decdf03c61f303fcfc82fe601beb4096d3305536
Author: Salvatore Orlando <email address hidden>
Date: Thu Apr 16 11:57:37 2015 -0700

    Quota model: use HasTenantId mixin

    This change simply changes the Quota model class to obtain
    the tenant_id from the mixin class. As the attribute in the
    mixin is identical to that in the model there is no need for
    a migration.

    This patch also removes a reference to quota classes in the
    docstring, as Neutron does not implement those. It is good
    to be careful when copying and paste code.

    Change-Id: Idab15d5ef2ddd2b830a7dcde46990506064535f7
    Closes-Bug: #1445169

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: neutron-pecan
Review: https://review.openstack.org/185072