Start adding reusable test fixtures.

Now that nova is using fixtures, we have grown some that can be used across
projects. The mox/stubout fixture is a great example of this - and also can
be used in the oslo tests themselves.

Change-Id: Ia78019317a6772c5c66a875c948854248383f9ea
Use testtools as test base class.

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
Merge "Start adding reusable test fixtures."
Merge "Use testtools as test base class."