Convert v3 config drive plugin to v2.1

This patch changes v3 config drive plugin 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: Ieed7619f3cb310b25d157ebb07f24468aea1af9d
Merge "Convert v3 config drive plugin to v2.1"