List Domains Test Case-V3

Adds a test script "test_domains.py" so as to test the list
domains functionality. And added required support functions
to identity_client.py of both v3/xml and v3/json interfaces.

Change-Id: I931be12bc2ea7a08b0272dfe145c13e195e69029
Implements: blueprint keystone-v3-domain-api-test
Merge "List Domains Test Case-V3"