[terraform] deploy landing on comm services cluster

Summary:
This puts up landing on our comm services cluster. This will require a new keyserver image with
https://phab.comm.dev/D12927

Depends on D12929

Test Plan: Tested with custom image and accessed landing page at comm.app

Reviewers: varun, bartek

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12930