[terraform] run keyserver isolated from webapp and landing

Summary:
Run keyserver nodes isolated by not running webapp and landing routers.

Depends on D12812

Test Plan:
terraform apply console logged keyserver.js and ensured that webapp and landing routers were not being set up but the keyserver router was properly setup

Reviewers: ashoat, varun, bartek

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12812