Backport some v3 aggregate API unittest to v2 API

This patch add some v3 aggregate API unittest into V2 API to improve
V2 API unittesting.

Change-Id: Id4d533e74b462b55b155b4e4ad7d94e018d35726
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 "Backport some v3 aggregate API unittest to v2 API"
Merge "Change v3 aggregate API to v2.1"