Change v3 flavor_rxtx to v2.1

This patch changes v3 flavor_rxtx API to v2.1 and makes v2
unit tests share between v2 and v2.1.

The differences between v2 and v3 are described on the wiki page
https://wiki.openstack.org/wiki/NovaAPIv2tov3.

Partially implements blueprint v2-on-v3-api

Change-Id: I0b79be078ff2b2ca682f1f061f40c81d4487c7de
Merge "Change v3 flavor_rxtx to v2.1"