Fix for OPENSTACK-939
Fix for OPENSTACK-939

Setting the gateway_ip value to 'None' when linking a
 VSD Managed subnet that was created with DHCP disabled.

Change-Id: I9f5b7cf2b012b5b1bc3671b5b6c7c59d328f8972
Closes-Bug: OPENSTACK-939

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