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.
> "our" chef-client is the one started by crowbar, right? Assuming that, you description is right. And at the end, we get an exception.

Yes.

> The question is how you can differentiate between a Exception from a failing chef-client (for whatever reason) and an exception because the node is rebooting. The latter one isn't really an error. 

The only thing I can think of is the exit code which might be different because ssh closes the connection.

> The proposed changes just skip the chef-run for nodes which are currently rebooting. The chef-run is done after the reboot by `crowbar_join` anyway.

Except that you won't know if the chef run will be successful, so you will report the proposal was successfully applied while it might not be true.

> Why do you want to run chef-client if you already know that it will fail?

I don't want to run it, I want to wait for the reboot to be over and then run it :-)

BTW, there's a race in your code because the attribute on the node about rebooting might not be set yet when you look for it.