We will need to have new API test in neutron-tempest-plugin to validate cascade network deletion.

Test can do something like:

1. Create network,
2. Create subnet(s) in the network,
3. Create router
4. Attach subnet(s) to the router,
5. Create some regular ports (can be unbound),
6. (optionally) create trunk port and subports,
7. Delete network with single API call with "cascade=True"
8. Validate that all resources were removed properly:
   1. all ports are deleted,
   2. trunk port and all subports are deleted,
   3. subnets are removed from the router,
   4. subnets are deleted
   5. network is deleted