[bug] Overlapping validation with nested subnets is bugged

[nemesifier]

When editing a subnet which has one or multiple level of children the overlap validation doesn't work as expected.

Here's how to replicate the issue:

• create a subnet 10.0.0.0/16 (call it master subnet)
• create a subnet 10.0.0.0/24 which has as master subnet the subnet created in the step above
• create a subnet 10.0.1.0/24 which has as master subnet the subnet created in the first step
• open the master subnet 10.0.0.0/16, change the description, hit save

When a subnet is

Expected result:

The change is saved.

Actual result:

ValidationError is raised:

Possible solution:

• first of all a failing test which replicates the bug should be written.
• then we need to make _validate_overlapping_subnets and make it exclude children, eg: qs = qs.exclude(pk__in=exclude).exclude(subnet=self.subnet, master_subnet=self, master_subnet__master_subnet=self)
• however, the solution proposed in the previous point will work only for 2 level nesting and not for more, so we have to find a solution that can traverse the available parents and if it finds that the last parent is itself the validation skips