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.
It still doesn't fix the race :-)

Here's the issue:
- user applies proposal
- chef-client daemon on node decides to run its periodic run; it's at its early stage, so hasn't reached the point where it knows a reboot will be needed
- we reach this code here
  - with your code, we call your `wait_for_reboot`: the attribute is not set yet, so nothing happens
  - we call the "ssh chef-client" bit; chef-client is blocked until the chef-client from the daemon exits
- chef-client from the daemon reaches the recipe bit where a reboot is required and sets the attribute
- chef-client from the daemon exits; our chef-client starts
- reboot happens; our ssh session is killed, so the call to `system(*ssh_cmd)` fails