Credentials Keystone V3 API Tests

This submission adds new test script 'test_credentials.py' so as
to verify CRUD Operations on Credentials API. And also added
required support functions in 'credentials_client.py' for both
JSON/XMl interfaces

Implements blueprint: add-credentials-test-keystone-v3

Change-Id: I00269eaeb01af821c47300a80d89fae3dd2598a2
Merge "Credentials Keystone V3 API Tests"