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.
```
time
---->

|--------------|
| Crowbar node | ------------------------------> |    proposal applied, trigger   | -----------------------------> | chef-client starts to run and fails |
|--------------|    apply proposal on compute      chef-client by crowbar via ssh      chef-client waits


|--------------|
| Compute node |----------------------> | chef-client run triggered  | -----------------------------------------> | chef-client finished - reboot triggered |
|--------------|                            by chef-client daemon       chef-client runs (and applies proposals)

```