Changes block_device_mapping extension into v2.1

This patch changes v3 block_device_mapping API to v2.1 and make
unittest 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: I935b336523241a32dbd2bb8b154ce72afefd3aa9
Merge "Changes block_device_mapping extension into v2.1"