add unit tests for the cdmi.py module

These are fine-grained tests, which test functions separately.
They are using nosetests, which seems easier to read than
python unittests itself.

The aim for these tests is to run automatically on commit
to check if something broke.
adapt the unit tests to the new database use

while this is fixed, the tests itself still fail,
because I can't get the registration_worker to be mocked.

If the function register_data_object is called directly
and not put in a new thread to work, mocking is fine.
With the thread creation, it fails.
correct the comment of get_dirlist_dict()