Port API contribs to Python 3

Port API contribs to Python 3

* Replace dict.iteritems() with dict.items(), dict.iteritems() was
  removed in Python 3.
* Replace dict.values() with list(dict.values()) to get a list on
  Python 3.
* HTTP body type must be bytes:

  - Fix unit tests to use bytes strings for HTTP body
  - Encode XML to UTF-8 on Python 3.

* Use "%r" instead of "%s" to format request and response in test to
  avoid BytesWarning on Python 3.
* tests-py3.txt: add cinder.tests.unit.api.contrib

Partial-Implements: blueprint cinder-python3
Change-Id: I65e412b2897635db5c8dfe13d61e71c52e0603e7