Added install:configuration tag to step that sets up forum env
When updating configuration settings on the Forum service, changed passwords for MongoDB were not being set if the Ansible playbook is targeting the `install:configuration` tag. Adding that tag to the `setup the forum env` step addresses this issue.
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.
[ ] 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)?