Port flavor_rxtx extension to v3 API Part 2

Ports the flavor_rxtx extension and the corresponding
unittests to the v3 framework.

Partially implements blueprint nova-v3-api

Change-Id: Id486e157e4f5ce467400c88ca89952d175f6e9a8
Merge "Port flavor_rxtx extension to v3 API Part 2"