Add username/password for Mongo login

[palfrey]

Description

Allows setting Mongo username/password from config (or environment variables via config) rather

Type of change

Please delete options that aren't relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please also list any relevant details for your test configuration

Checklist

• Updated documentation if needed
• Tests added/amended
• bazel test //... passes locally
• PR is contained in a single commit, using git amend see some docs

---

This change is