Provide yum specific image building glue.

RHEL/Fedora/CentOS and other yum based distributions need specific yum
customizations.

Customizations include caching of downloaded yum packages outside of the build
chroot so that they can be reused by subsequent image builds.  The cache
increases image building speed when building multiple images, especially on
slow connections.  This is more effective than using an HTTP proxy as a yum
cache since the same rpm from different mirrors is often requested.

A custom yum repository configuration can also be applied by defining
`DIB_YUM_REPO_CONF` to the path to a repo configuration file. The file will
be copied to /etc/yum.repos.d/dib-yum-repo-conf.repo during the image build,
and then removed at the end of the build.