cobbler: Support CentOS Stream

cobbler: Support CentOS Stream

"But David, why aren't we just using the CentOS 8 Stream ISO?"

Thank you for asking.
1) The vmlinuz and initrd that are baked into the CentOS 8 Stream ISO as of this writing are not compatible.  Filed https://bugs.centos.org/view.php?id=18188
2) Even if I take the vmlinuz and initrd from CentOS 8.3, after firstboot, the system still bizarrely boots as CentOS 8.3.  That's right, when using the CentOS 8 Stream ISO and we kickstart using Cobbler, you end up with CentOS 8.3.  I'm sure it has something to do with our kickstarts and snippets but nothing obvious stood out to me.

Signed-off-by: David Galloway <dgallowa@redhat.com>