Added 2 user related testcases for Keystone V3API

This submission adds two test cases namely 'test_user_update' and
'test_list_user_projects' in script test_users.py. The suporting
methods are added to identity_client.py for JSON and XML calls.

Change-Id: I0a66b0c40bc275f4c0da3f57ea9c9d33cdeea455
Implements: blueprint keystone-v3-users-api-test
Merge "Added 2 user related testcases for Keystone V3API"