dnf: remove files from installed packages

This is a workaround for a current dnf bug, it doesn't update the state
of the packages after they are installed so we tear down the base dnf
object and create a new one pointing to the installroot.

There is an additional issue with the list of files returned, hawkey and
dnf don't appear to make a distinction between files, dirs and ghosted
dirs like yum did, this can result in too much being removed (eg. all of
/etc/selinux/) so we only remove files not directories.