Improve gateway_ip updation for subnet

Allow update 'gateway_ip' to null or empty for
neutron subnet, and keep the same behavior as
creation.
Also, this change adds some related functional
tests for neutron subnet resource.

Closes-Bug: #1510800
Change-Id: Ib5511402e1d118457356a8add0042be2b157eb24
1 file changed
tree: 0967830a3c0389d7ba603cae52742fac3d4e7ab3
  1. common/
  2. functional/
  3. scenario/