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
283c9705c0