yum-minimal: add systemd to initial install

It seems in the grub cleanup in
Iafe3611f4eec3c6357587a6cae6a30a261686ead I managed to unintentionally
drop systemd from the yum-minimal builds.  By not pre-installing grub
we dropped some dependencies; the path is tortured ... grub2 ->
os-prober -> udev -> systemd-udev -> systemd (we don't even want
os-prober!  So this whole thing was working by accident).

This manifests in *very* confusing ways.

Currently centos-minimal builds are failing late in the build with
services unable to enabled.  dib-init-system was actually trying to
tell us that it didn't know what init was installed (because systemd
wasn't actually installed), but unfortunately it was not really
failing.  This meant the service files were not copied correctly from
other elements, and thus fail to be enabled.  I have corrected this
with I076c08190d40c315ad6a6d96a3823e9fc52630be which would at least
alert us earlier.

For Fedora 24, due to a bug in dracut dependencies [1], missing the
systemd-udev package fails the build of the initrd during the kernel
install.  This then results in an initrd-less, unbootable system (see
also Ibaaa81124098f3c6febe48e455d3e1cd0a5f1761).

Add these dependencies explicitly.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1398505

Change-Id: I24ce648485c3d6f3c27ab8f87a638516b3727017

elements/yum-minimal/root.d/08-yum-chroot

@@ -242,7 +242,17 @@ else
     sudo -E chroot $TARGET_ROOT ${YUM} -y update
     sudo -E chroot $TARGET_ROOT ${YUM} -y \
         --setopt=cachedir=/tmp/yum/$ARCH/$DIB_RELEASE \
-        install passwd findutils sudo util-linux-ng
+        install systemd passwd findutils sudo util-linux-ng
+
+    # This package is split out from systemd on >F24, dracut is
+    # missing the dependency and will fail to make an initrd without
+    # it; see
+    #  https://bugzilla.redhat.com/show_bug.cgi?id=1398505
+    if [ $DISTRO_NAME = "fedora" -a $DIB_RELEASE -ge 24 ]; then
+        sudo -E chroot $TARGET_ROOT ${YUM} -y \
+            --setopt=cachedir=/tmp/yum/$ARCH/$DIB_RELEASE \
+            install systemd-udev
+    fi
 
     # Put in a dummy /etc/resolv.conf over the temporary one we used
     # to bootstrap.  systemd has a bug/feature [1] that it will assume