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.
> @vuntz a chef-client run is triggered after the reboot by `crowbar_join`.

And? What's the issue with that? If we call chef-client, then it will just wait for the first chef-client to complete.