Inherit nat attribute from network to subnet in same zone

[hknutzen]

If network S is subnet of network N and both are located inside the same security zone, we assume that S is part of N.
Currently attribute 'owner' of N is inherited to S.
But attribute 'nat' of network N isn't inherited to S.
This is inconsistent.

This is a bit difficult to change, because the subnet relation is only known after find_subnets_in_zone is called, which currently also checks NAT information.