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.

[rjschwei]

Given that this is not a functionality change and more of a code cleanliness item let's make sure this does not get merged until after 2.17 is cut. We agreed 2.17 testing will start tomorrow and we do not want to interfere with the agreed upon timeline by merging more changes that force testing to start over.

[rjschwei]

I will come back for review after 2.17 is done.