QoS API and DB models

* Implements a vendor neutral database model centered around a QoS
object.
	* QoS objects have a type and policy.
		* Type is an Enum, with two current choices: "DSCP" for
		marking packets with a DSCP mark, and "ratelimit" which will
		throttle a port or network based on a bandwidth value
		* The policy field will store either the DSCP mark
		that will be applied (as a string), or the bandwith
		value in kbps (as a string).

* A concrete implementation will be written for the OpenVSwitch plugin,
to support QoS via DSCP marking, as well as bandwidth limiting.

* Other plugins can extend the database model with more columns if
required

* QoS objects can be linked to Networks or Ports

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

Blueprint quantum-qos-api-db

Change-Id: Ia4d5c6cae82c579928129d0728cb03091d2869a7
[WIP] OpenVSwitch QoS support

implements bp qos-ovs-qos

Change-Id: Ib0e2bed8763cbca8e75ced8b2ba57c7c548ff719