QoS API and DB models

* Implements a vendor neutral database model centered around a QoS
object, that has a policy field that can store arbitrary JSON.

* QoS objects can be linked to Networks, Subnets, or Ports.

* QoS mappings to a port overrides a subnet policy, subnet
policy overrides a network policy. If no policy is specified,
the policy is inherited from the parent object.

* Needs more unit tests

Blueprint quantum-qos-api-db

Change-Id: Ia4d5c6cae82c579928129d0728cb03091d2869a7