compose: Tell yum not to cache packages locally
compose: Tell yum not to cache packages locally

They get deleted anyways since we blow away the tmpdir, but we expect
people to be using a local HTTP proxy.  We stopped trying to cache
packages internally as that caused conflicts with the yum lock with
concurrently executing composes.

mdshuai/rpm-ostree,tmp-user/rpm-ostree,giuseppe/rpm-ostree,tmp-user/rpm-ostree,mdshuai/rpm-ostree,cgwalters/rpm-ostree-cgwalters,cgwalters/rpm-ostree-cgwalters,cgwalters/rpm-ostree-cgwalters,mdshuai/rpm-ostree,giuseppe/rpm-ostree,tmp-user/rpm-ostree,mdshuai/rpm-ostree,giuseppe/rpm-ostree,giuseppe/rpm-ostree,tmp-user/rpm-ostree