[terraform] Dont load-balance EC2 services in staging

Summary:

This stack resolves issues encountered when setting up fresh staging AWS account with Terraform.

On staging we don't have legacy EC2 services, so these data blocks errored because couldn't find them.
Made them conditional.
Task for deleting legacy EC2 instances: ENG-4543 - worth doing later after everything is confirmed to be working.

Test Plan: terraform plan for production doesn't change anything, for staging it doesn't error.

Reviewers: jon, varun

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8712