Fix Python 3 issues in targets unit tests

Fix Python 3 issues in targets unit tests

* Replace StandardError with ZeroDivisionError: StandardError was
  removed in Python 3.
* Replace __builtin__ with six.moves.builtins.
* Open text mode with "w" mode (instead of "wb").
* tox.ini: add targets unit tests to Python 3.4

Note: cinder.tests.unit.targets.test_iet_driver still fails on Python 3,
it will be fixed in a different change.

Blueprint cinder-python3
Change-Id: Ie0f5d2dfaf2ffdeab29fe40f692a2f09fb5a7aba