Change v3 keypairs API to v2.1

This patch changes v3 keypairs 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: I1fcb93a00816eab2d87f251b7cf8a8992e920308
Merge "Change v3 keypairs API to v2.1"