Fix field name for neutron_subnet in case of NSX

'gateway' field for neutron_subnet type was changed to
'gateway_ip' in commit 247ea68f2. But the NSX module is using this filed
by an old name.

Change-Id: Ibfefdc7bd79fb64906d78cb79bdf44127b17f46f
Closes-Bug: 1389317
Merge "Fix field name for neutron_subnet in case of NSX"