On the path to testr migration, we need to replace the unittest base classes
with testtools.
Replace tearDown with addCleanup, addCleanup is more resilient than tearDown.
The fixtures library has excellent support for managing and cleaning
tempfiles. Use it.
Replace skip_ with testtools.skipTest
Part of blueprint grizzly-testtools.
Change-Id: I45e11bbb1ff9b31f3278d3b016737dcb7850cd98