Add a CCIManager.get_instance_passwords(id) method

Returns a list of passwords attached to the instance, or an empty list if none were found. These don't come back from `get_instance`, for some reason, and the API considers the password part of the `get_instance` object mask to be valid.

Would be helpful to have the same method in the hardware manager too (issue #54), since they both use the same `getOperatingSystem` API call to retrieve passwords.

Includes a test that passes.