Workaround the problem with subnet4-del and subnet6-del and shared networks

The issue is related to the Kea bug: kea#3455 (closed)

Suppose you have a shared network and a subnet within this shared network. Both the shared network and the subnet are associated with two servers. Edit the subnet. Deselect the shared network and remove one of the daemons from the subnet. Submit the form.

The expected outcome is that the subnet is promoted to a top-level subnet on one of the servers (removed from the shared network), and completely removed from the other server. It looks like it is the case, but if you open raw configuration of the server from which the subnet should be removed, it is still there under the shared network. This is a bug in Kea, but we can work around it in Stork by sending the network4-subnet-del or network6-subnet-del command before deleting the subnet.