Fix superfluous GET requests in swift-bench.

If you specified num_gets = 0 for a benchmarking run (say, if you're
benchmarking PUT rate), you'd still get each swift-bench-client
process doing 1 GET request. Now you don't.

This should also fix the case where you've got more objects than
swift-bench-client processes, for example when you're uploading a few
large objects and then doing lots of parallel GETs of those
objects. Now you'll get the number requested, not
max(number-requested, number-of-bench-clients).

Change-Id: Ied9eb733dd9af51a3c6af8b815ad6cff0ff746b7