Add packagegroup and image recipe for minimal SELinux image.

This is intended to demonstrate the minimal set packages necessary
to boot and load a system with SELinux enabled. Specifically we
don't need any of the packages that depend on python.
Add packagegroup for policycoreutils packages.

The policycoreutils package previously included most everything in
the base package. This packagegroup is intended to fill the role
of the old policycoreutils package and pull in all packages from the
policycoreutils recipe.