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.
apologies but I think there's a much simpler and cleaner way to fix this. Together with https://github.com/crowbar/barclamp-provisioner/pull/324 we just have to check **before** and **after** we run the chef-client via ssh if we need to wait until a reboot is done. Currently we just check this **after** the chef-client run.