Trial using pubsub in sandbox

Added 10 minute ack deadline.
Note that for 50MB tar files with typical distribution of around 40 tests per file, the processing takes under a minute. However, for very slow tests, the trace files are small, so there may be 100s of tests in a tar file, and it may take more like 5 minutes to process. We will need to monitor whether some test files are resubmitted, resulting in higher dup rates.

Also, note that AppEngine appears to do autoscaling only if there are HTTP requests passed through the load balancer. Since subscription method uses pulls directly from the pubsub system, AppEngine does not autoscale. We will need to use a different scaling mechanism.