Fixed error in setUpClass method from PythonDockerTestMixin class.
Fixed error in setUpClass method from PythonDockerTestMixin class.

ERROR: setUpClass (__main__.MyTests)
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Traceback (most recent call last):
  File "/home/treemo/.virtualenvs/python_docker_test/lib/python2.7/site-packages/python_docker_test/mixin.py", line 115, in setUpClass
    raise cls.container_start_thread.exc_info[1], None, cls.container_start_thread.exc_info[2]
AttributeError: type object 'MyTests' has no attribute 'container_start_thread'

harvard-dce/canvas-docker-testing