heat engine : Don't wait() for killed greenthreads
heat engine : Don't wait() for killed greenthreads

We don't care about the return value of any killed threads on
delete_stack, and the greenthreads will all raise a GreenletExit
exception after being kill()'d anyway.
Also see related openstack-common fix : https://review.openstack.org/15906

Signed-off-by: Steven Hardy <4d1810d1ff6d7f22d774341c4549c02173d2f2ab@redhat.com>

redhat-openstack/heat,rh-s/heat,takeshineshiro/heat,ntt-sic/heat,noironetworks/heat,cwolferh/heat-scratch,Triv90/Heat,citrix-openstack-build/heat,ntt-sic/heat,miguelgrinberg/heat,NeCTAR-RC/heat,openstack/heat,cryptickp/heat,maestro-hybrid-cloud/heat,pratikmallya/heat,steveb/heat,citrix-openstack-build/heat,dragorosson/heat,JioCloud/heat,steveb/heat,noironetworks/heat,varunarya10/heat,jasondunsmore/heat,varunarya10/heat,pratikmallya/heat,jasondunsmore/heat,srznew/heat,rickerc/heat_audit,rdo-management/heat,dims/heat,dims/heat,srznew/heat,rdo-management/heat,pshchelo/heat,rickerc/heat_audit,NeCTAR-RC/heat,JioCloud/heat,cryptickp/heat,miguelgrinberg/heat,rh-s/heat,gonzolino/heat,openstack/heat,Triv90/Heat,takeshineshiro/heat,cwolferh/heat-scratch,gonzolino/heat,redhat-openstack/heat,pshchelo/heat,maestro-hybrid-cloud/heat,dragorosson/heat,Triv90/Heat