[keyserver] Make it possible to configure Redis host

Summary:
Today we always hit localhost. I'm working on Dockerizing the keyserver, and since we're using docker-compose we will need to allow this to be configurable.

Depends on D3849

Test Plan: in concert with D3851, I tested that yarn update-geoip worked inside the Docker image (it accesses Redis, admittedly for no good reason)

Reviewers: palys-swm, jimpo

Reviewed By: palys-swm

Subscribers: Adrian, atul, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3850