Implement Registry BBM config for Omnibus

Summary

We recently added "Batched Background Migrations" (BBM) configuration support to the Helm chart. We should add the same functionality to the Registry service configuration and testing.

Details

Related Create and Deploy Registry Database Batched Bac... (&13609), we have implemented BBM for the registry (&15338 (closed)). In order to roll this functionality out, we need to add this to the Omnibus GitLab.

Actionable

• Registry configuration of Batched Background Migrations has been added to Omnibus
• Testing of this configuration, and possibly functionality, has been added to the Omnibus RSpec