[keyserver] Allow listening address to be configured

Summary:
In order to be accessed from the Docker host, we need to be able to tell the Node server to listen for incoming connections from outside localhost.

Depends on D4172

Test Plan: Without this, I was not able to access Node from my host machine. Afterwards I was able to get a response

Reviewers: palys-swm, atul

Reviewed By: atul

Subscribers: Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4173