Centralized region->policy mapping

[roman-khimov]

Is your feature request related to a problem? Please describe.

I'm always frustrated when we can't easily have some way to synchronize a set of S3 gateways for the region->policy map. While #989 is an improvement, it's still a local one, each node has a mapping of its own.

Describe the solution you'd like

Have some way to fetch policies by name, it can be:

• some specific contract with known API
• NNS
• Authbox container with special objects

Exact method tbd.

Describe alternatives you've considered

Keep it per-node, have fun synchronizing configurations.