[terraform] Replace all config options with terraform variables and remove sops

Summary:
Bartek pointed out that SOPS was overkill for self-hosting. Additionally, several configuration options required the user to make terraform changes.

In this diff, I introduce the usage of terraform variables to

1. configure mariadb username/password that was previously configured through sops
2. configure the aws region, subnets, and user ip

Depends on D12323

Test Plan: terraform apply and successfully connected mysql client to rds endpoint

Reviewers: bartek, varun, ashoat

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12324