[SECURITY] configure via env vars

[jblachly]

Presently, chamberlaind is configured via command line params. Unfortunately, this means that any user irrespective of privilege level can see the entire command line, including RPC passwords for bitcoind and chamberlaind.

One way to mitigate this is to configure the application alternatively via environment variables. This has the additional benefit of fitting more nicely into typical configuration procedures for container orchestration systems such as k8s, nomad, and AWS ECS.