Change v3 extended_server_attibutes to v2.1

This patch changes v3 extended_server_attibutes API to v2.1 and makes v2
unit tests share between v2 and v2.1.
Revert v3 changes back to v2.
os-extended-server-attributes:* -> OS-EXT-SRV-ATTR:*
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: I02b203b62b9e675f71d0ff3880bc9ee051a3b80d
Merge "Change v3 extended_server_attibutes to v2.1"