Adds test for list/get volume attachments V2 APIs

This patch adds the test for following Nova V2 APIs-
- List volume attachments
- Get volume attachment
Response Schema has been added for the same.

To make consistent in server_client and easy for tests to compare
the response, this Patch also change the returned response body from
server_client's attach_volume().

Note- XML test cases is not implemented as it is deprecated.
New test cases has been skipped for XML.

Change-Id: Iee055ceaad135bba4a57b010d1e08652e2d13598
Merge "Adds test for list/get volume attachments V2 APIs"