labs: sleep in shutdown_controller.sh

After building the cluster with the two additional snapshots on the
controller VM, we end up with a dying nova-compute service.

Sleeping 2+ seconds after booting into compute and before rebooting
controller fixes the problem: nova-compute keeps running and its status
in nova-manage service list will go to XXX and back to :-) without user
intervention.

Change-Id: I8ba20c77392b0966d6febfc300790cf1943f2ad5
Merge "labs: sleep in shutdown_controller.sh"