No longer define libvirt_cpu_mode

If the libvirt_virt_type was qemu, we'd set it to none and otherwise
pass an undefined variable to puppet-nova.
The default from puppet-nova is sane, let's rely on it instead.

Change-Id: I9baf60ca04765d096bb1dc8ecafafba9adefa9df
(cherry picked from commit df00bfcf1a)

packstack/puppet/templates/nova_compute_libvirt.pp

@@ -5,7 +5,6 @@ Firewall <| |> -> Class['nova::compute::libvirt']
 
 if str2bool($::is_virtual) {
   $libvirt_virt_type = 'qemu'
-  $libvirt_cpu_mode = 'none'
 } else {
   $libvirt_virt_type = 'kvm'
 }
@@ -33,7 +32,6 @@ $libvirt_vnc_bind_host = hiera('CONFIG_IP_VERSION') ? {
 
 class { '::nova::compute::libvirt':
   libvirt_virt_type        => $libvirt_virt_type,
-  libvirt_cpu_mode         => $libvirt_cpu_mode,
   vncserver_listen         => $libvirt_vnc_bind_host,
   migration_support        => true,
   libvirt_inject_partition => '-1',