Port volume_attachments extension to v2.1 API

This patch ports v2 volume_attachments & volume_attachment_update
extension to v2.1.
Unittest code modified to share testing with both v2/v2.1.
API sample file and their tests have been added.

Partially implements blueprint v2-on-v3-api

Change-Id: Icf63529f317bc61debc665641d1c9d9f2e45bcec
Merge "Port volume_attachments extension to v2.1 API"