SE-2321 Makes the mongo authentication mechanism configurable from the environment

Description

Allows the mongo client's `auth_mech` to be configured from the environment and via ansible.

*JIRA tickets*: OSPR

*Discussions*: [Discussion forum errors in latest master with mongo 3.2](https://discuss.openedx.org/t/discussion-forum-errors-in-latest-master-with-mongo-3-2/1843/5)

*Sandbox URL*: TBD - sandbox is being provisioned.

*Merge deadline*: ASAP, fix needed for Juniper.

*Testing instructions*:

1. Login to the sandbox
1. Visit the edX Demo course's Discussion forum
1. Ensure you can post to the forum with no errors.
1. Visit a unit with a discussion inline unit.
1. Ensure you can post to the forum with no errors.

*Reviewers*

  • [ ] @toxinu

  • [ ] edX reviewer[s] TBD

FYI @bderusha

*Settings*
```yaml
FORUM_MONGO_AUTH_MECH: mongodb_cr
```

Assignee

Bill DeRusha

Reporter

Open Source Pull Request Bot

Labels

Contributor Name

Jillian Vogel

Repo

edx/cs_comments_service

Customer

Epic Link

None

OSCM Assignee

None

Platform Map Area (Levels 1 & 2)

Developer Experiences

Platform Map Area (Levels 3 & 4)

None

Blended Hour Utilization Percentage

None

Priority

Unset
Configure