Add tests for server-group Nova V2 APIs

This patch adds the JSON tests for following server-group Nova V2 APIs
1. server-group create
2. server-group delete
3. server-group list
4. server-group get

It Also adds the clients for JSON

Closes-Bug: 1297933
Change-Id: Iee2717872d58a715f9e012218a67bf040d2abe80
Merge "Add tests for server-group Nova V2 APIs"