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 for forgetting one small bit: can we change this to like "if test -f /var/chef/cache/chef-client-running.pid ; then flock -n /var/chef/cache/chef-client-running.pid true ; fi"? I don't think we should fail if the file or directory doesn't exist, or we can't write to it, or whatever.