Change v3 extended_status to v2.1

This patch changes v3 extended_status API to v2.1 and makes v2
unit tests share between v2 and v2.1.
Revert v3 changes back to v2.
os-extended-status:* -> OS-EXT-STS:*
The differences between v2 and v3 are descibed on the wiki page
https://wiki.openstack.org/wiki/NovaAPIv2tov3.
Partially implements blueprint v2-on-v3-api

Change-Id: Ieefca1d0e8da831830f43cc83eb50bc7e7f4a10c
Merge "Change v3 extended_status to v2.1"