Prior to this commit, the install_package and uninstall_package methods
used Yum for all versions of Red Hat Enterprise Linux. However, RHEL has
used DNF as its default package manager since version 8.

This commit updates those methods to use DNF for RHEL 8, 9, and any
future versions and to explicitly use Yum on RHEL versions 1-7.

Prior to this commit, the install and uninstall methods for Fedora
defaulted to using DNF as its package manager for versions greater than
or equal to 22 but less than 40, and Yum for every other version.

This commit configures Beaker to use DNF for every version of Fedora, as
DNF has been the default package manager for Fedora for the last 8
years.