Addition of XML support to test_quotas.py

Added logic to test_quotas.py file so as to support XML calls.
Hence added exclusive client files for XML. Also modified JSON client
file, manager.py and clients.py according to new addition of
XML support.

Change-Id: Ibbdd87c2d1bb4e26ff0f77441dc91d71abc00d7f
Implements: blueprint add-xml-support
Merge "Addition of XML support to test_quotas.py"