Port API to Python 3

Port API to Python 3

* test_make_flat_dict(): only use one dictionary key. On Python 3,
  the hash function is now randomized by default and so dictionary
  keys are returned in a random order.
* Replace "not cmp(a, b) == 0" with "a != b"
* test_common: replace range(n) with list(range(n)) to get a list on
  Python 3. On Python 3, range() now returns an iterator.
* Replace dict.items()[0] with list(dict.items()[0]). On Python 3,
  dict.items() now returns a view which is not indexable.
* VolumeTypesController.index(): replace dict.values() with
  list(dict.values()) to get a list on Python 3.
* test_xmlutil: use byte strings for XML
* test_extensions: use byte strings for HTTP body
* Add the following tests to tests-py3.txt:

  - cinder.tests.unit.api.test_common
  - cinder.tests.unit.api.test_extensions
  - cinder.tests.unit.api.test_versions
  - cinder.tests.unit.api.test_xmlutil

Partial-Implements: blueprint cinder-python3
Change-Id: I40c28f6b6a1cf72ce13774755153c8691b4d621b