Adds environment variable support to mongodb uri string

#### What's this PR do?

Makes the mongod string environment aware

#### Where should the reviewer start?

The config index file.  You will see I added conditional logic to look for an environment variable set to `DB_URI`

#### How should this be manually tested?

Set a local environment variable named `DB_URI` to a dummy string value.  Run the app and watch it burn.

#### Any background context you want to provide?

This is need to make the app play nice in all environments (nodejitsu).

#### Release Type:
- [ ] Major
- [ ] Minor
- [X] Patch