Consistently use dj-database-url and encourage postgres

After reviewing #123, some additional tasks are clearer:

in #123, dj-database-url updates the `DATABASES` variable if the environment variable `DATABASE_URL` is set. If not set, the default of a local sqlite3 file comes through. We don't want to encourage sqlite3 for the reasons outlined in #119.

- [ ] Remove the sqlite3 config from `bespin/settings.py`
- [ ] Document how to construct a database URL and consider providing a postgres default.
- [ ] Remove the [overrides from `bespin/settings_prod.py`](https://github.com/Duke-GCB/bespin-api/blob/eaecbf741a7a0c284148f41e1c132168339e70b2/bespin/settings_prod.py#L25-L33) that elect a postgres database and configure with independent environment variables.
- [ ] Update the ansible role to construct and provide the database url
- [ ] Optional - Bail out of settings.py if no database_url is found, with instructions on how to set that.