[terraform] use .env in self-host folder for keyserver config variables instead of specifying values in terraform.tfvars or terraform.tfvars.json

Summary:
This addresses https://linear.app/comm/issue/ENG-8844/make-terraform-config-add-task-def-variables-through-a-env-file

We use a local .env file (functionally identical to one we would use in comm/keyserver/.env) to include the env variable for the primary/secondar node ecs tasks

Originally, we were using terraform.tfvars to specify the values for these environmental variables and hardcoding a set list of env variables that would be included in our primary/secondary ecs tasks.

The new method allows the user to specify any env variable which will then appear in the ecs task by modifying the .env file in comm/services/terraform/self-host/.env

Test Plan:
terraform apply. Configuration is working and successfully populates the task def. New tasks launch and run successfully without error from startup.
If tasks were unable to read these environmental variables (i.e. COMM_JSONCONFIG_secrets_user_credentials), the tasks would fail.

Depends on D12731

Reviewers: varun, ashoat, bartek

Reviewed By: varun, ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12800