Convert v3 certificate API to v2.1

This patch changes v3 certificate 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: I8e6556dffbc14334f7ec1b9db38fbbfe2387bef0
Merge "Convert v3 certificate API to v2.1"