Test cases for Roles V3 Actions

This submission adds 'test_roles.py' test script with test cases
so as to verify the functionality of all 'role' actions available
in V3 keystone api. And also with required support functions in
identity_client.py of both JSON/XML interfaces

Change-Id: I79f0a68ab62e7d86b5789e5e6f938621b35cf70b
Implements: blueprint keystone-v3-roles-test
Merge "Test cases for Roles V3 Actions"