Need to rewire provider-specific tests in the storage charm

After refactoring the code in the storage charm to be a little more straightforward, the tests had to be unwired.

They need to be rewired to the top-level MAkefile "test" target somehow.

Challenges:

I think the rewiring is the easy part of the problem: the provider-specific unit tests should probably live in the storage-providers.d/ subfolder, but that isn't a python module.
- We can always hack away, but running the tests there (cd storage-providers.d/blah/ && nosetests) will break imports since the common code is not in $PYTHONPATH in that case, and making "storage-providers.d/" a python module doesn't work either (import storage-providers.d.nfs.whatever ? See the problem dot there? :) )
- Another option would be to put the provider-specific tests in the top-level directories... but in this case we can't import the things we're trying to test.

So I guess we need to tweak $PYTHONPATH in the test runner invocation to include the provider-specific folders in $PYTHONPATH, then imports should by "jumping" into storage-providers.d/, like "from nfs.hooks import blah" (skipping storage-providers.d altogether).