temp: Enable Celery distributed tracing in stage edxapp

[timmc-edx]

Datadog Support suggested that we try enabling this to see if it changes anything about the orphaned spans on the edxapp workers (unexpected top-level spans that are not operation_name:celery.run, but instead are other spans that have lost their parent association).

Just going to enable this on stage so we don't mess with traces in prod, for now.

See edx/edx-arch-experiments#822

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Make sure that the following steps are done before merging:

• Have a Site Reliability Engineer review the PR if you don't own all of the services impacted.
• If you are adding any new default values that need to be overridden when this change goes live, update internal repos and add an entry to the top of the CHANGELOG.
• Performed the appropriate testing.