Port targets test_iet_driver to Python 3

Port targets test_iet_driver to Python 3

* Replace StandardError with MemoryError: StandardError was removed
  in Python 3
* Replace __builtin__ with six.moves.builtins to mock open()
* Mock the logger in test_create_iscsi_target() because the test
  injects errors in open() and the logger uses open() to format the
  traceback
* tox.ini: add cinder.tests.unit.targets.test_iet_driver to Python 3.4

Partial-Implements: blueprint cinder-python3
Change-Id: I7c83da7374ef78cc98bd6c0c5596b7a7adb70303