Support Bandwidth limiting QoS support

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Neutron has supported Bandwidth limiting QoS support since Liberty. Horizon needs to add support for Bandwidth limiting QoS support and their use to expose this functionality.

Motivation
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Neutron bandwidth limiting qos configurations.The main motivation for this implementation is about limiting, prioritizing or guaranteeing speed of traffic . So we can process important user traffic with higher priority and cloud admin can offer different service levels based on the available network bandwidth. Qos policies can be applied per network and port levels upon where cloud admin wants to get applied.

Description :
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QoS design defines the following two conceptual resources,

[1] QoS policy
[2] QoS rule (type specific)

Each QoS policy contains zero or more QoS rules. A policy is then applied to a network or a port, making all rules of the policy applied to the corresponding resource.When applied through a network association, policy rules could apply or not to neutron internal ports (like router, dhcp, load balancer, etc..). The QosRule base object provides a default should_apply_to_port method which could be overridden. In the future we may want to have a flag in QoSNetworkPolicyBinding or QosRule to enforce such type of application (for example when limiting all the ingress of routers devices on an external network automatically).

Data Model Impact:

The model consist of two main parts, policies, and rules. Policies are composed of rules. Even if that can look overcomplicated as a start, it will allow the feature to be easily extended without changes to object relationships (QoSPolicy <-> Port / QoSPolicy <-> Network) in the future.

For the case of bandwidth limiting, we have a third model QoSBandwidthLimitRule that extends QoSRule base data model.

New rules could be introduced by adding new submodels to QoSRule.

UX
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The nature of this change is wiring in Horizon. The end user should able to create QOS rule using Horizon UI as same neutron supports CLI interface .

Doc Impact
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The doc impact of this change will be to the overall openstack documentation regarding domain management.