Fix network_gateway residue when resource deletion

Should set resource_id after network_gateway creation
instead of after network_gateway connection for
OS::Neutron::NetworkGateway, to avoid the network_gateway
residue when the resource deletion.

Change-Id: I89e1aed10685c0448a15decb95b89dbd946d39c2
Closes-bug: #1368008
Merge "Fix network_gateway residue when resource deletion"