Create subnet without gateway and explicit IP ver

Now it's impossible to create subnet without gateway in network
tests. This patch allows you to set gateway explicitly to None.
Backward compatibility is supported: by default it creates
subnet with default gateway as before. Also it adds possibility
to create subnet with specific IP version when you need to create
two subnets in one tenant of different IP version ("dual-stack").
Fixed attributes test for new requirements and added 2 anothers.

Change-Id: I7aca5e07be436f20cba90339785b46182d97fead
Merge "Create subnet without gateway and explicit IP ver"