Replace custom tearDown with fixtures and cleanup.

testtools.addCleanup is a more resilient way to perform cleanup activities,
as it will continue to clean things up even if there are unforseen problems.

Specifically, replace custom management of tempdirs with fixtures.TempDir
and replace tearDown methods that can be easily replaced with calls to
addCleanup in the setUp method. There are at least two temp dir creations that
did not have a corresponding cleanup in this patch, which is another reason
for using useFixture(fixtures.TempDir) instead of calls to mkdtemp.

Part of blueprint grizzly-testtools.

Change-Id: I4eb548010612bd5a8d30e8e2304fa66d3d5ffb7c