Convert v3 cells API to v2.1

This patch changes v3 cells API to v2.1 and makes v2
unit tests share between v2 and v2.1.

This patch also delete the V3 cells API tests.

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: Iacfd1881ff24eb3bfc73006244044fc139897eda
Merge "Convert v3 cells API to v2.1"