Fix Python 3 issues in cmd

Fix Python 3 issues in cmd

* Replace filter() with a list-comprehension using if to get a list on
  Python 3.
* Get the mock module from the stdlib unittest module on Python 3.3 and
  newer, or fallback to the third-party mock module.
* Replace __builtin__ with six.moves.builtins.
* tox.ini: add the following tests for Python 3.4

  - cinder.tests.unit.test_api
  - cinder.tests.unit.test_cmd

Blueprint cinder-python3
Change-Id: Iea516ae598e8eebfc1087663a9b3e0a00d0633d3