Clean up QoS rules first, then QoS policies

Clean up QoS rules first, then QoS policies

Since policy deletion kills all rules by CASCADE, cleaning rules up
after policies is basically no-op. Instead, let's swap the order and
in that way implicitly cover rule deletions.

Change-Id: Id3a7d8ddf5599a532e3d5609d94522579f85b938
Partially-Implements: blueprint quantum-qos-api