Change v3 aggregate API to v2.1

This patch changes v3 aggregate 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: I95372d49ffacd12bdc8e2f6b60628c17f70d0eba
Merge "Change v3 aggregate API to v2.1"