[Tunnelbroker] implement WNS config

Summary:
WNS config for tunnelbroker, similar to keyserver config

Depends on D12931

Test Plan: Add config var from commandline and log it

Reviewers: kamil, bartek, marcin

Reviewed By: kamil, bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12932