Check for existence of snat port before deleting it
Check for existence of snat port before deleting it

Delete logic was the last place where such check was missed.

Change-Id: I9fb2916af1b5087b4e6268c040b00404c431e288
Closes-Bug: #1579259

cloudbase/neutron,openstack/neutron,huntxu/neutron,mahak/neutron,eayunstack/neutron,bigswitch/neutron,huntxu/neutron,sebrandon1/neutron,noironetworks/neutron,mahak/neutron,igor-toga/local-snat,eayunstack/neutron,noironetworks/neutron,sebrandon1/neutron,bigswitch/neutron,openstack/neutron,openstack/neutron,igor-toga/local-snat,mahak/neutron,cloudbase/neutron