Fix missing subscription after product migration

[ngetahun]

Description

Fix missing subscription from activation during 15sp5 -> 15sp6 migration.

related card: https://trello.com/c/SN979OIx/3274-rmt-client-activation-subscription-gone-after-migration-from-15sp5-to-15sp6

Change Type

Please select the correct option.

• [x] Bug Fix (a non-breaking change which fixes an issue)
• [ ] New Feature (a non-breaking change which adds new functionality)
• [ ] Documentation Update (a change which only updates documentation)

Checklist

Please check off each item if the requirement is met.

• [x] I have verified that my code follows RMT's coding standards with rubocop.
• [x] I have reviewed my own code and believe that it's ready for an external review.
• [ ] I have provided comments for any hard-to-understand code.
• [ ] I have documented the MANUAL.md file with any changes to the user experience.
• [x] RMT's test coverage remains at 100%.
• [x] If my changes are non-trivial, I have added a changelog entry to notify users at package/obs/rmt-server.changes.

Other Notes

Please use this space to provide notes or thoughts to the team, such as tips on how to review/demo your changes.

[ngetahun]

The PR looks good to me code wise. But I have to admit that I am not sure exactly how to test it locally. How would you do it? Which steps can you recommend to test this properly?

I can setup a test server. Test instructions are in trello card.