Environment variables for db connection configs

#### What's this PR do?
Database connection settings can be configured through environment variables. Practically what this allows for is for `.env.*.php` files to provide the settings needed depending on the environment. For local workspaces, a developer would modify their `.env.local.php` file. For production, it'd be `.env.php`.

#### What are the relevant tickets?
#11 

@codal-sara @angaither