oslo.policy can not handle policy file removal. As a result, if policy
overrides were defined at some point, but then removed, causes service
outage.
While we could add a handler trigger to restart the service
on policy removal, it's better to simplify the logic and always place an
empty policy.yaml even if no overrides are defined.
Change-Id: Ia51d88d64cc268fc1c44082aa28b235250392c16
Signed-off-by: Dmitriy Rabotyagov <dmitriy.rabotyagov@cleura.com>