add a check prior to the execution of "yum update" to see if the update ...

I saw the pull request for your fix to add "yum update" as "upgrade=yes", and that it was rejected for not checking first to see if there was a need for package updates.  I've added what I think mpdehaan that check to your existing code via the yum command check-update.

I am not a python developer, so if you see anything wrong with my code or style, feel free to tell me what to do differently.