Port WSGI tests to Python 3

Port WSGI tests to Python 3

* Replace dict.keys()[0] with list(data.keys())[0]. On Python 3,
  dict.keys() now returns a view which is not indexable.
* Skip SSL tests on Python 3. Tests hang for an unknown reason, they
  must be fixed later.
* Fix unit tests: HTTP body type is bytes, not Unicode.
* Debug.print_generator(): on Python 3, write into sys.stdout.buffer
  instead of sys.stdout, because HTTP body type is bytes not Unicode.
* ResponseObject: encode serializer output to UTF-8 if it's Unicode.
* tox.ini: add the following tests to Python 3.4

  - cinder.tests.unit.api.openstack.test_wsgi
  - cinder.tests.unit.wsgi

Note: Ignore pylint error E1101 on sys.stdout.buffer. pylint on
Python 2 complains that the buffer attribute doesn't exist, whereas
the code is only executed on Python 3 and the attribute exists on
Python 3.

Related-Bug: #1505103
Partial-Implements: blueprint cinder-python3
Change-Id: I0db0e04010e41be71192a2e4db13829114ad6eef