[tunnelbroker] Allow configuring AMQP credentials

Summary:
We need a way to provide RabbitMQ credentials, without providing them in AMQP URI, since AWS doesn't expose authenticated URI.
Also, made AMQP URI configurable via env vars

Test Plan: Ran external RabbitMQ with custom credentials and configured tunnelbroker using environment variables

Reviewers: kamil, michal, varun, jon

Reviewed By: kamil, michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9153