fedostree/products: Use fedora-rawhide-kernel-nodebug by default

The system is too painful to use otherwise.
postprocess: Always place rpmdb into /usr/share/rpm

Remove support for the case where the tree doens't have rpm - we don't
need to go quite that small yet, and having the file -> package
mapping helps with the GPL compliance story.

See https://lists.fedoraproject.org/pipermail/legal/2014-February/002393.html
autobuilder: Inherit comment attribute into treefiles
fedostree/products: Fully SELinux enforcing
fedostree: Rename to "fatomic"