Add optional task to rebuild the forum elasticsearch index

Description

This change is required for deployments maintain an persistent database of forum posts, but run ElasticSearch on localhost, so that the index is recreated with each openedx appserver.

Set FORUM_REBUILD_INDEX to run the rebuild_index rake task.

*JIRA tickets*: OSPR

*Merge deadline*: Prior to the Gingko.RC1 cut, if possible. CC @nedbat

*Testing instructions*:

1. Snapshot your devstack, for safety.
1. As a user with sudo access (e.g. `vagrant`), run:
```
source /edx/app/edx_ansible/venvs/edx_ansible/bin/activate
cd /edx/app/edx_ansible/edx_ansible/playbooks/
ansible-playbook -i localhost, -c local \
-e '{"migrate_db": "yes"}' \
-e '{"role": "forum", }' \
run_role.yml
```
1. Note that `[forum : initialize elasticsearch]` is run, but `[forum : rebuild elasticsearch indexes]` is skipped.
1. Now run:
```
ansible-playbook -i localhost, -c local \
-e '{"migrate_db": "yes", "FORUM_REBUILD_INDEX": "yes"}' \
-e '{"role": "forum" }' \
run_role.yml
```
1. Note that `[forum : initialize elasticsearch]` is run, then `[forum : rebuild elasticsearch indexes]` is run.

*Reviewers*

  • [ ] @mtyaka

  • [ ] edX reviewer[s] TBD

Configuration Pull Request

Make sure that the following steps are done before merging:

  • [ ] A DevOps team member has approved the PR.

  • [ ] Are you adding any new default values that need to be overridden when this change goes live? 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.

  • [ ] Have you performed the proper testing specified on the [Ops Ansible Testing Checklist](https://openedx.atlassian.net/wiki/display/EdxOps/Ops+Ansible+Testing+Checklist)?

Done

Assignee

Unassigned

Reporter

Open Source Pull Request Bot

Contributor Name

Jillian Vogel

Repo

edx/configuration

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