Convert v3 flavor extraspecs plugin to v2.1

This patch changes v3 flavor extraspecs plugin 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: Ibf42ad56ead33846d57904dde2a07f721e6e4af3
Merge "Convert v3 flavor extraspecs plugin to v2.1"