modify nova.conf.erb for allinone install in vm

When nova boot a vm in one VM, it will failed.
We set 'libvirt_cpu_mode=none' and
'libvirt_use_virtio_for_bridges=false' to make it
work well.

Change-Id: Ia517d5af4aeed0f42c04a4ff0de6ca4f3a53575a
Closes-Bug: #1228609
Merge "modify nova.conf.erb for allinone install in vm"