QoS core extension: fixed dict extension when QoS policy is unset

QoS core extension: fixed dict extension when QoS policy is unset

Previously, if QoS policy was detached from a core resource, core
resource extension didn't update the resource dict with the new QoS
policy value (None), and that resulted in no notification sent to the
agent about the change, so QoS rules were not flushed from the affected
ports.

Change-Id: I22397af3a43254d146abaa4a4429ac654b4c3c50
Partially-Implements: quantum-qos-api