Tolerate missing user on signal responder delete
Tolerate missing user on signal responder delete

SignalResponder delete fails if the created user
has already been deleted.

This change catches the 404 as for the User resource.

Change-Id: I55034469d10f2742296f0415fe26272d3683530d
Closes-bug: #1211633

rickerc/heat_audit,jasondunsmore/heat,citrix-openstack-build/heat,openstack/heat,pshchelo/heat,cryptickp/heat,noironetworks/heat,JioCloud/heat,rickerc/heat_audit,maestro-hybrid-cloud/heat,noironetworks/heat,jasondunsmore/heat,cwolferh/heat-scratch,dragorosson/heat,takeshineshiro/heat,varunarya10/heat,citrix-openstack-build/heat,rh-s/heat,dragorosson/heat,gonzolino/heat,rh-s/heat,srznew/heat,ntt-sic/heat,NeCTAR-RC/heat,openstack/heat,cryptickp/heat,varunarya10/heat,steveb/heat,pratikmallya/heat,steveb/heat,rdo-management/heat,miguelgrinberg/heat,dims/heat,pshchelo/heat,dims/heat,maestro-hybrid-cloud/heat,cwolferh/heat-scratch,redhat-openstack/heat,gonzolino/heat,pratikmallya/heat,takeshineshiro/heat,srznew/heat,NeCTAR-RC/heat,rdo-management/heat,miguelgrinberg/heat,JioCloud/heat,redhat-openstack/heat,ntt-sic/heat