Fix for OPENSTACK-1351
Fix for OPENSTACK-1351

Fix the code which updates the gateway IP and allocation pools of an
existing subnet.

Change-Id: I0cb8d752a8464a8635584956d49f1c681332bbaf
Closes-Bug: OPENSTACK-1351

naveensan1/nuage-openstack-neutron,nuagenetworks/nuage-openstack-neutron,naveensan1/nuage-openstack-neutron,nuagenetworks/nuage-openstack-neutron