DRY and make database.yml more flexible

This enables us to fix
https://github.com/openfoodfoundation/ofn-install/issues/387 and brings the
needed flexibility so things like
https://github.com/openfoodfoundation/openfoodnetwork/pull/3887 or any CI don't
require a custom version. That's what ENV vars are for!

For instance, I no longer need to mess with my LXC setup to connect to the DB.
I just need to have 2 env vars.

config/database.yml

@@ -1,19 +1,23 @@
-default: &default
+defaults: &defaults
   adapter: postgresql
-  encoding: utf8
+  encoding: unicode
   pool: 5
-  timeout: 5000
-  host: db
-  port: 5432
+  host: localhost
+  username: <%= ENV.fetch('DB_USERNAME', 'ofn') %>
+  password: <%= ENV.fetch('DB_PASSWORD', 'f00d') %>
 
 development:
-  <<: *default
+  <<: *defaults
   database: open_food_network_dev
-  username: ofn
-  password: f00d
 
 test:
-  <<: *default
+  <<: *defaults
   database: open_food_network_test
-  username: ofn
-  password: f00d
+
+production:
+  <<: *defaults
+  database: open_food_network_prod
+
+staging:
+  <<: *defaults
+  database: open_food_network_prod