Add is_fedora and exit_distro_not_supported functions

Between is_fedora, is_ubuntu and is_suse, we can make the code a bit
simpler to read. We also use exit_distro_not_supported to identify
places where we need implementation details for new distros.

As "/sbin/service --skip-redirect" is Fedora-specific, guard this with a
is_fedora test too.

Change-Id: Ic77c0697ed9be0dbb5df8e73da93463e76025f0c
diff --git a/lib/nova b/lib/nova
index 3a4d34d..8272ef0 100644
--- a/lib/nova
+++ b/lib/nova
@@ -394,11 +394,13 @@
 function install_nova() {
     if is_service_enabled n-cpu; then
         if is_ubuntu; then
-            LIBVIRT_PKG_NAME=libvirt-bin
+            install_package libvirt-bin
+        elif is_fedora || is_suse; then
+            install_package libvirt
         else
-            LIBVIRT_PKG_NAME=libvirt
+            exit_distro_not_supported "libvirt installation"
         fi
-        install_package $LIBVIRT_PKG_NAME
+
         # Install and configure **LXC** if specified.  LXC is another approach to
         # splitting a system into many smaller parts.  LXC uses cgroups and chroot
         # to simulate multiple systems.