[Keyserver] Use getCommConfig for tunnelbroker connection

Summary:
Allow for the tunnelbroker endpoint to be configured. Necessary
for targeting production or staging instance during deployments.

https://linear.app/comm/issue/ENG-4706

Depends on D8956

Test Plan:

cd keyserver
yarn dev # Should say defaulting to local instance
export COMM_JSONCONFIG_facts_tunnelbroker="{\"url\":\"ws://127.0.0.1:51001\"}"
yarn dev # Should omit default message, still connect to local instance

Reviewers: marcin, jon, ashoat

Reviewed By: marcin

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8957