Change v3 flavors to v2.1

This patch changes v3 flavors 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: I1e52c034a8c50fde3aa31e1e79c8f113424fd2e1
Merge "Change v3 flavors to v2.1"