Fix for openstack-617
Fix for openstack-617

Admin tenant can list vlans using gateway and gatewayport option.
Just providing gateway should return an error message to neutronclient.

Change-Id: I3b6b156e94140dc0b74afd01efd58addffb9998e
Closes-Bug: #openstack-617

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