Can't shelf a cluster that was created from deleted shelf

[tobifroe]

I ran into a weird issue yesterday - it seems like currently it's impossible to create a shelf from a cluster that was itself created from a shelf that has been deleted in the meantime. Steps to reproduce:

1. Create shelf from any cluster
2. Restore shelf to a new cluster
3. Delete shelf
4. Shelf newly restored cluster (from step 2)
5. -> Errors (see operator log attached):

[tschale]

We probably should delete the fromShelf from the beiboot CRD, after the cluster became ready. I suspect that the cluster reconcile also won't work when a shelf is deleted.