Fix for OPENSTACK-1609
Fix for OPENSTACK-1609

Using subnetmapping only
it the mapping actually exists.

Change-Id: I68c5d54aa8fecfba9c6d29ce8e56f77db6c1e9ea
Closes-Bug: OPENSTACK-1609

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