(QENG-3919) Return 503 status on failed allocation

Making sure we go back to the original functionality, which was:

 - status 200 when vms successfully allocated
 - status 404 when a pool name is unknown
 - status 404 when no pool name is specified
 - status 503 when vm allocation failed

lib/vmpooler/api/v1.rb

@@ -104,6 +104,7 @@ module Vmpooler
       if failed
         vms.each do |(template, vm)|
           return_single_vm(template, vm)
+          status 503
         end
       else
         vms.each do |(template, vm)|