Change v3 agents API to v2.1

This patch changes v3 agents 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: I96dd9cd3672c6d584bd0019c385544c08f687e7f
Merge "Change v3 agents API to v2.1"