Bug 1296047 - Vagrant: Avoid repetition in DATABASE_URL_RO
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

KWierso/treeherder,akhileshpillai/treeherder,akhileshpillai/treeherder,kapy2010/treeherder,tojonmz/treeherder,tojon/treeherder,glenn124f/treeherder,tojonmz/treeherder,tojonmz/treeherder,glenn124f/treeherder,tojonmz/treeherder,edmorley/treeherder,glenn124f/treeherder,tojon/treeherder,glenn124f/treeherder,KWierso/treeherder,KWierso/treeherder,edmorley/treeherder,edmorley/treeherder,tojon/treeherder,glenn124f/treeherder,akhileshpillai/treeherder,kapy2010/treeherder,tojonmz/treeherder,akhileshpillai/treeherder,tojon/treeherder,KWierso/treeherder,akhileshpillai/treeherder,kapy2010/treeherder,glenn124f/treeherder,kapy2010/treeherder,tojonmz/treeherder,kapy2010/treeherder,edmorley/treeherder,akhileshpillai/treeherder