Add ability to use service accounts in MongoDB

Currently, we are using MongoDB as root (not passing credentials when creating a `MongoClient` instance). This might pose some security issues, as the account used by the service should have restricted access (i.e. read only, read/write, but not delete).

We need to add credentials to configuration file, update deployments when needed.

Start by locating the MongoClient creation and the configuration used by deployments; the payload names no files or tests. Trace how MongoDB settings are supplied, then determine how restricted service-account roles should be represented and verify that deployments authenticate without granting unnecessary access.

Written by the indexing model from the issue text.