Fix config-dir for middleware tests

The etc directory from pycadf 0.5 is installed into the virtualenv created
by tox at the top, not inside the library directory. Adjust the location
the audit middleware tests use to find the directory.

Cherry-pick of 530bcd8071dfd9dba3ca08d54b0e8a5900231631

Change-Id: I01268c6294ed8799622300a0db01fc836a2f544f