Add cpu_mode in vagrant/bootstrap.sh

With libvirt 2.0, when using qemu cpu mode is not properly
evaluated and need to be set to cpu_mode = none.

Add this option when kvm is not supported, otherwise user
will still facing errors when launching instances over qemu.

Change-Id: I7aa6df356085263d25c6afcc812f035d3ded942b

contrib/dev/vagrant/bootstrap.sh

@@ -205,7 +205,8 @@ EOF
     mkdir -p /etc/kolla/config/nova/
     cat > /etc/kolla/config/nova/nova-compute.conf <<EOF
 [libvirt]
-virt_type=qemu
+virt_type = qemu
+cpu_mode = none
 EOF
 
     # Launch a local registry (and mirror) to speed up pulling images.