policy: fix policy_file finding

Currently, the policy.json file is only looked inside current directory.
Let's use cfg.config_dir via the find_file method to find the policy file.

Also add some unit test to catch theses errors.

This fixes bug #1098206

Change-Id: I896108f1bd66d3be6ec32b50d9dddc6c368f3709
Signed-off-by: Julien Danjou <julien@danjou.info>