Nova boot will fail if the host doesn't have libvirt installed

Libvirt contains a package that sets the permissions of KVM.
If the host does not have libvirt installed the permissions
will never be set and nova boot will fail.

Change-Id: Ie5509199391db55e0984d59a6623f021f3ae46c1

docker/nova-compute/nova-libvirt/start.sh

 #!/bin/sh
+
+# If libvirt is not installed on the host permissions need to be set
+chmod 660 /dev/kvm
+chown root:kvm /dev/kvm
+
 echo "Starting libvirtd."
 exec /usr/sbin/libvirtd