update kernel task for centos 8
Modular kernel packaging (https://fedoraproject.org/wiki/Changes/Modular_Kernel_Packaging_for_Cloud) is no longer just a Fedora thing -- it is in CentOS 8. It would be nice to update the kernel task so that a list of packages can be specified, to be copied to the nodes and fed to "rpm -i" together.
I say nice because we will continue building a single package using mainline rpm targets (as we always did), so this shouldn't block CentOS 8 rollout in the lab. However, installing a kernel built in koji/brew (or locally using Fedora/CentOS rpm targets instead of mainline rpm targets) is no longer possible because "kernel" is a meta package which requires two other packages: "kernel-core" and "kernel-modules".