make sure fixed version of yum (not affected with bsc#1221223) is installed

[zpetrova]

Description

Please describe your change and provide as much context as possible.

Newest yum update released last week break our repository (cannot read repomd.xml) (bsc#1221223) This script makes sure fixed yum is installed.

Fixes # (issue) (bsc#1221223)

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.

• [ ] 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.
• [ ] 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.

My changes are trivial, so I think changelog entry is not necessary, but nyway I wrote changelog entry.

Other Notes

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