[Geo][Registry Replication] Fix the source of notification secret for omnibus

What does this MR do?

After first testing of Docker Registry Replication using omnibus package, I've found that settings work not in a way I expected. Notification setting should be passed to rails section as we don't have access to the registry section

The changelog is not needed as the feature is not released yet.

The counterpart MR omnibus-gitlab!3549 (merged)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team