Don't use a query for stack_get()
Don't use a query for stack_get()

When getting a database record by ID, it is much more efficient to get() it
directly than to perform a query or the database to obtain the ID (again)
and then get it.

Since stack_get() returns only a single record that can be trivially
checked for deletion in code, do this rather than performing a query with a
filter.

Change-Id: I4bb3ea1a9c4ae928fe0f5bce5c02b842a3f1ab4f

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