Do not download policies that have already been downloaded

[flavio]

Right now policy-server will always download all the policies defined inside of the policies.yml file, even if they have already been downloaded.

This is sub-optimal, changing a policy setting requires to restart the policy-server, which then causes all the policies to be downloaded again.

Acceptance criteria

• policies.yml file is extended to allow something similar to Kubernetes' imagePullOptions
• policy-server follows the policy pullOption and downloads the wasm file only when needed