[terraform] Localstack changes for commtest

Summary:
Minor changes to localstack terraform configuration.
They're needed to be able to successfully destroy resources using terraform apply -destroy.
Also made localstack endpoint configurable - this will be required later when running terraform from within a container.

Test Plan:

• TF plan shows no changes on remote config
• On local dev config, doing

cd services; docker compose up -d localstack
cd terraform/dev
terraform apply -auto-approve
terraform apply -destroy -auto-approve

successfully initializes and then destroys all resources.

Reviewers: varun, michal, kamil, jon

Reviewed By: michal, kamil

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9507