QoS API and DB models

* Implements a vendor neutral database model centered around a QoS
object.
	* QoS objects have a type
		* Type is an Enum, with two current choices: "DSCP" for
		marking packets with a DSCP mark, and "ratelimit" which will
		throttle a port or all ports attached to a network based on a bandwidth value
	* QoS objects have policy key-value pairs for implementation specific behaviors

* 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
flake8

Change-Id: I64685d3da9506cf965d7db204048bf3540c20f54