Move ZypperAuth.verify_instance to InstanceVerification engine

[jesusbv]

Description

Move the instance verification check to the InstanceVerification engine

Change Type

Please select the correct option.

• [ ] 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.
• [ ] I have documented the MANUAL.md file with any changes to the user experience.
• [X] RMT's test coverage remains at 100%.
• [ ] 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.