Change v3 hosts to v2.1

This patch changes v3 hosts 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: I8ce66570edb06bde0bdea677e6e784539aaab763
Merge "Change v3 hosts to v2.1"