Enable access control and authentication on MongoDB

[netoax]

The MongoDB is currently unprotected. In this way, if you access the mongo shell in your host machine using just the raspberry IP, you can operate on it and drop a database.

$ mongo <raspberry_ip>/knot_web
> db.dropDatabase()
> use knot_fog
> db.dropDatabase()

To solve this issue we need to follow the MongoDB Security Checklist, specially to enable the minimal security configuration: authentication and access control mechanisms.