use filesystem driver for API conformance tests

Related to:#388 (closed)

This MR adds the ability for a filesystem driver to be used for the API integration tests. Since we'll want to expand the general conformance tests to include migration environments, and test specific migration behavior, we'll need to use a driver that supports the rootdirectory parameter and has persistence (so, anything other than inmemory), since migration behaviors may rely on these properties to be tested.