Expire registry tokens in 8 hours, or as configured

[bear454]

Description

Registry engine creates JWT tokens for authz/n to an affiliated container registry. The tokens should expire in 8 hours, unless configured otherwise. Issuance is condition on the system having been 'last_seen' within the token expiration period.

Change Type

Please select the correct option.

• [x] Bug Fix (a non-breaking change which fixes an issue)
• [x] 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.
• [x] I have provided comments for any hard-to-understand code.
• [x] 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.

[jesusbv]

Closing this PR, with the new cache system, this PR is not needed