Setting dop to zero does not raise an error

While I did some testing (with automatic topology plugging code) I set the dop of all spouts and bolts to zero. I submitted the topology to LocalCluster and did not get any error.

It took me a while to figure out that the wrongly specified dop were the reason for an empty result of the test. From a user point of view, it would be nice to raise an exception when the parallelism_hint is smaller than 1.