Add RHEL 8 composer support

[mglantz]

https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8-beta/html/installing_and_deploying_rhel/building-custom-system-images-with-composer_graphical-installation

[mglantz]

What this means is that you install cockpit, cockpit-composer, composer-cli and lorax-composer packages. This gives you access to the image composer in RHEL8, when released.