Add negative UT cases for subnet/GW create/update

When force_gateway_on_subnet is configured, ensure
that a GW with IP same as subnet, and BCAST GW IP
are rejected during subnet create. For subnet update,
ensure GW outside of subnet is rejected.

bug 1178675

Change-Id: Ice58f7e1be7966b247c165e52807405401e5a4ea
Merge "Add negative UT cases for subnet/GW create/update"