Add auth to the MongoDB database for production environment

### Objective
Add a user and password for the MongoDB database to have a better security in production.

### Checklist
- [ ] Add a user and password with comments in the `.env.example` file
- [ ] Add the auth variables to the connection strings in the `application-prod.yml` file
- [ ] Add the auth variables to the connection strings in the `app.yml` file
- [ ] Update the readme to show how we should run the `app.yml` in local to inject the secrets in the `.yml` files
- [ ] Update the `cd.yml` with the new user and password variables.

> **Note:** The `application-dev.yml` file should be handle without user and password, the same way the `mongodb.yml` configuration