libvirt: Support the use of the virt-preview repo when using Fedora

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

Author: lyarwood

lib/nova_plugins/functions-libvirt

@@ -24,6 +24,10 @@ DEBUG_LIBVIRT=$(trueorfalse True DEBUG_LIBVIRT)
 # Currently fairly specific to OpenStackCI hosts
 DEBUG_LIBVIRT_COREDUMPS=$(trueorfalse False DEBUG_LIBVIRT_COREDUMPS)
 
+# Enable the Fedora Virtualization Preview Copr repo that provides the latest
+# rawhide builds of QEMU, Libvirt and other virt tools.
+ENABLE_FEDORA_VIRT_PREVIEW_REPO=$(trueorfalse False ENABLE_FEDORA_VIRT_PREVIEW_REPO)
+
 # Enable coredumps for libvirt
 #  Bug: https://bugs.launchpad.net/nova/+bug/1643911
 function _enable_coredump {
@@ -61,6 +65,12 @@ function install_libvirt {
         #pip_install_gr <there-si-no-guestfs-in-pypi>
     elif is_fedora || is_suse; then
 
+        # Optionally enable the virt-preview repo when on Fedora
+        if [[ $DISTRO =~ f[0-9][0-9] ]] && [[ ${ENABLE_FEDORA_VIRT_PREVIEW_REPO} == "True" ]]; then
+            # https://copr.fedorainfracloud.org/coprs/g/virtmaint-sig/virt-preview/
+            sudo dnf copr enable -y @virtmaint-sig/virt-preview
+        fi
+
         # Note that in CentOS/RHEL this needs to come from the RDO
         # repositories (qemu-kvm-ev ... which provides this package)
         # as the base system version is too old.  We should have