Xml Support for Image Test scripts

Added/Modified required support functions in images_client.py
XML interface script.Added required logic to below scripts
inorder to support both JSON and XML interfaces
* tests/compute/images/test_image_metadata.py
* tests/compute/images/test_list_image_filters.py
* tests/compute/images/test_list_images.py
* tests/compute/test_authorization.py

Change-Id: Ibcce0d81024b46c1b031bc348ca8e8d62e005d8d
Implements: blueprint add-xml-support
Merge "Xml Support for Image Test scripts"