Device is not removed from ClusterConfiguration, if respective Peer is deleted

[mbfm]

Currently, it is possible to delete a Peer which has one of its devices included into a ClusterConfiguration.
This means that deploying the cluster fails, because no respective Peer can be identified for the given device. This error is only visible in the CARL logs.

Tasks:

• [ ] Test this to ensure that this is really what happens.
• [ ] Decide whether we want to block deleting Peers which are included into a Cluster, or handle this differently.