Makes the forum authentication mechanism configurable

*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)

*Dependencies*: https://github.com/edx/cs_comments_service/pull/316

*Sandbox URL*: See https://github.com/edx/cs_comments_service/pull/316

*Merge deadline*: ASAP, fix needed in Juniper

*Testing instructions*:

See https://github.com/edx/cs_comments_service/pull/316

*Reviewers*

• [ ] @toxinu

• [ ] edX reviewer[s] TBD

FYI @bderusha

Configuration Pull Request
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Make sure that the following steps are done before merging:

• [ ] A DevOps team member has approved the PR if it is code shared across multiple services and you don't own all of the services.

• [x] Are you adding any new default values that need to be overridden when this change goes live? No, default value leaves behaviour unchanged.
  If so:

• [ ] Update the appropriate internal repo (be sure to update for all our environments)

• [ ] If you are updating a secure value rather than an internal one, file a DEVOPS ticket with details.

• [x] Add an entry to the CHANGELOG.

• [ ] If you are making a complicated change, have you performed the proper testing specified on the [Ops Ansible Testing Checklist](https://openedx.atlassian.net/wiki/display/EdxOps/Ops+Ansible+Testing+Checklist)? Simple change.
  Adding a new variable does not require the full list (although testing on a sandbox is a great idea to ensure it links with your downstream code changes).

• [ ] Think about how this change will affect Open edX operators. Have you updated the wiki page for the next Open edX release?
  Will post info on [Juniper Confluence](https://openedx.atlassian.net/wiki/spaces/COMM/pages/940048716/Juniper).