Reset yum/dnf cache to correct location

Make sure we reset the yum/dnf cache to /var/cache/${YUM}, not just
/var/cache/yum

This was resulting in the F22 fedora-minimal image being larger than
the base-image.  Because F22 fedora-minimal does some installs with
dnf when bootstrapping the chroot before we set "cachedir=" to the
bind-mounted external cache, we have "/var/cache/dnf" created and and
populated with the package meta-data, etc.

When we globally point dnf to /var/cache/yum here, we effectively
orphan the /var/cache/dnf created in those first steps.  dnf doesn't
care, but we end up with two copies of all the package metadata, etc
in "/var/cache/dnf" & "/var/cache/yum".

This also cleans up the sed a bit, by just replacing the lines.

Change-Id: Icc98fe30c34cb941aed4b987647ab67ac34af15a

elements/yum/post-install.d/99-reset-yum-conf

@@ -8,9 +8,11 @@ set -o pipefail
 
 if [[ $DISTRO_NAME == "fedora" && $DIB_RELEASE -ge 22 ]]; then
     cfg=/etc/dnf/dnf.conf
+    cachedir=/var/cache/dnf
 else
     cfg=/etc/yum.conf
+    cachedir=/var/cache/yum
 fi
 
-sed -i 's/keepcache=1/keepcache=0/' $cfg
-sed -i 's/cachedir=\/tmp\/yum/cachedir=\/var\/cache\/yum/' $cfg
+sed -i "/^keepcache/c\keepcache=0" $cfg
+sed -i "/^cachedir/c\cachedir=${cachedir}" $cfg