Test script for CRUD operations on regions

Adds test_regions.py test script to identity v3.
Also adds related XML and JSON supporting methos in client file
region_client.py. Modified the identity base.py and common file
clients.py with details about region_client.
The test cases in the test script are
test_create_update_get_delete_region,
test_create_region_with_specific_id and
test_list_regions.

Change-Id: Ib0f07d04f6c31729bacdcfde065777e64fb76333
Merge "Test script for CRUD operations on regions"