Test middleware test_faults to Python 3

Test middleware test_faults to Python 3

* XMLDictSerializer: sort dictionary items to get a reliable XML
  output. On Python 3, dictionary items are iterated in a random
  order, because the hash function is randomized by default.
* Use byte strings for HTTP body
* On Python 3, decode serialized data to get Unicode
* Add a base TestCase class to factorize the _prepare_xml() method
* tests-py3.txt: add cinder.tests.unit.api.middleware.test_faults

Partial-Implements: blueprint cinder-python3
Change-Id: I81cd509230265f47f8c00ade14c71f66c717ca81