Change v3 flavor_access to v2.1

This patch changes v3 flavor_access 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: I75891e76753fba937b4a01f592afb4bd03aacd9c
Merge "Change v3 flavor_access to v2.1"