Fix get_subnet when the network is external and the sharednetworkresource is deleted on VSD.
Fix get_subnet when the network is external and the sharednetworkresource
is deleted on VSD.

Change-Id: Icab017459bb3e009db92429144bca1e80f32e38b
Closes-Bug: OPENSTACK-1562
(cherry picked from commit 412ac0a6280e7f9364dfea140d27e0aaa09cc248)

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