Port API v1 and v2 to Python 3

Port API v1 and v2 to Python 3

* Replace jsonutils.dumps() with jsonutils.dump_as_bytes() since
  output is used for the HTTP body, and HTTP body type is bytes, not
  Unicode. On Python 3, jsonutils.dumps() returns Unicode.
* FakeHttplibSocket: replace io.StringIO with io.BytesIO, HTTP body
  type is bytes. Encode Unicode to UTF-8.
* Fix FakeHttplibSocket.makefile() API: only the first parameter is
  mandatory. On Python 3, it's only called with the first 'mode'
  parameter.
* Use bytes strings to test HTTP bodies (JSON or XML).
* Replace dict.values() with list(dict.values()) to get a list on
  Python 3.
* Replace error.message with encodeutils.exception_to_unicode(error),
  exceptions loose their message attribute in Python 3.
* Update JSON serializer test in test_wsgi.py to use bytes.
* tests-py3.txt: add cinder.tests.unit.api.v1 and
  cinder.tests.unit.api.v2

Partial-Implements: blueprint cinder-python3
Change-Id: I0f0048f4a1344feaa3434cbf7ebd31e3f12d6ae4