Bug 1296047 - Vagrant: Avoid repetition in DATABASE_URL_RO

By using django-environ's proxied environment variable feature, like is
already being used in Heroku's value for `DATABASE_URL_RO`. See:
https://github.com/joke2k/django-environ/blob/v0.4/environ/environ.py#L257-L260

puppet/manifests/vagrant.pp

@@ -33,7 +33,7 @@
 export ENABLE_LOCAL_SETTINGS_FILE='True'
 export BROKER_URL='amqp://guest:guest@localhost//'
 export DATABASE_URL='mysql://${DB_USER}:${DB_PASS}@localhost/treeherder'
-export DATABASE_URL_RO='mysql://${DB_USER}:${DB_PASS}@localhost/treeherder'
+export DATABASE_URL_RO='$DATABASE_URL'
 export ELASTICSEARCH_URL='http://localhost:9200'
 
 export TREEHERDER_DEBUG='True'