Change lsbmajdistrelease to operatingsystemmajrelease for better compatibility

Under EL, lsbmajdistrelease is only valid if redhat-lsb or the equivalent is installed. This is a poor assumption especially in cloud environments which rely on minimal installs. Adding redhat-lsb to fix the problem means ~60 extra packages and ~90 MB, or we could just use $::operatingsystemmajrelease... :)