Fix race with multiple chef-client runs

Filter out all nodes which are currently rebooting to fix a possible race.
The race is possible when a proposal is applied *and* then the chef-client
daemon triggers a run.
This leads to the situation that the client run (triggered by crowbar)
waits until the other chef-client run (triggered by the daemon) finished
and then the chef-client (triggered by crowbar) starts - but the other
chef-client (triggered by the daemon) already triggered the reboot.
I guess I'm failing to understand what's happening. Could you do some ascii art to explain the events?

To me, what's happening is that:
- a chef-client is already happening
- our chef-client blocks; it does indeed reset ththe attribute about reboot, but it's irrelevant anyway because our chef-client is interrupted by the reboot and therefore exits with non-0, which means we don't wait for the reboot
- we try again with the `badones` thing, and since the node is rebooting, it doesn't work