Test cases for Policy V3 API-New

Adds a test script "test_policy.py" so as to test all the policy
API actions. And policy_client.py is added with all the required
methods. Implementation done in JSON and XML interfaces.

Change-Id: I1f4de6f0d9b8bfd58da3fd14819f3173a3f5adba
Implements: blueprint keystone-v3-policy-api-test
Merge "Test cases for Policy V3 API-New"