fixes #6126 - deleting a subnet that is associated with a host group throws foreign key error

From a networking perspective this isn't quite right, as "10.0.0.3/255.255.255.3" isn't valid (or is at least a very weird subnet). It should probably be something like:
```
sequence(:network) {|n| "10.0.#{n}.0" }
mask '255.255.255.0'
```