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 BadRequest tests of v2 quota_sets API

In test_quotas, there are BadRequest tests of v2 quota_sets API.
Most parts of them are duplicated.
This patch merges them for the readability and clarifying their purposes.

Change-Id: I4313b2a80819260e9363f457ba271689308b145f
Merge "Change v3 flavor_rxtx to v2.1"
Merge "Merge BadRequest tests of v2 quota_sets API"