Since commit a2d84f10e09baaec844386b4fb204b8bfe75ffd0 was merged,
horizon uses openstacksdk instead of neutronclient for network/subnet
API calls. Update the test data structure to simulate the one returned
by openstacksdk.
Change-Id: I3e5dea7fa3e382c06e8d7bd3c85e384ca1348af8