Favor addCleanup() over tearDown()

If setUp() fails, meaning that tearDown() is not called,
then any cleanup functions added will still be called.

Note: any tempory files are now cleaned up by the NestedTempFile
      fixture in oslotest.
https://github.com/openstack/oslotest/blob/master/oslotest/base.py#L37

Change-Id: I665de130b703e3aa111d53e7078722099ce91144
Merge "Favor addCleanup() over tearDown()"