Unit tests for Mistral benchmark are not running in py27 gate

__init__.py file is missed in the rally/tests/unit/benchmark/scenarios/mistral directory.

Also, after adding this file and running tests user will receive following error:
Traceback (most recent call last):
  File "/home/akuznetsova/COMMIT/rally/.tox/py27/local/lib/python2.7/site-packages/mock.py", line 1201, in patched
    return func(*args, **keywargs)
  File "tests/unit/benchmark/scenarios/mistral/test_workbooks.py", line 39, in test_create_workbook
    mistral_scenario.create_delete_workbook(definition)
AttributeError: 'MistralWorkbooks' object has no attribute 'create_delete_workbook'