fix: configurable xqueue tele backend

[jansenk]

see https://github.com/edx/edx-internal/pull/12199 and openedx/xqueue#966

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.