The virt-preview repo provides the latest rawhide versions of QEMU,
Libvirt and other virt tools for older releases of Fedora. This repo is
extremely useful when testing features in OpenStack that rely on these
latest builds well in advance of them landing in full Fedora, CentOS or
RHEL releases.
This change adds a ``ENABLE_FEDORA_VIRT_PREVIEW_REPO`` configurable
to control when this repo is enabled and used when deploying on Fedora.
Change-Id: I5c3e1b7b632fd73310c462530990cdb0e0c0ceea
169f5dee47