Change v3 os-user-data extension to v2.1

This patch changes v3 os-user-data extension 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: Ia45398c2ec7ff40851f94260dc10c53766581ff4
Merge "Change v3 os-user-data extension to v2.1"