spark: Don't hardcode the number of workers
spark: Don't hardcode the number of workers

Before, the Spark test assumed that there would be 4 worker machines.
Because commit 8a69a13 decreased the number of worker machines to 3, the
Spark test failed because of a placement rule that forbade placing two
Spark containers on the same machine.

We now determine the number of Spark workers to deploy based on the
`infrastructure.nWorker` variable.

kelda/kelda,kelda/kelda,LuiseV/quilt,kklin/quilt,quilt/quilt,LuiseV/quilt,kklin/quilt,quilt/quilt,kayousterhout/quilt,ejj/quilt-dev,LuiseV/quilt,NetSys/quilt,LuiseV/quilt,LuiseV/quilt,kklin/quilt,kelda/kelda,NetSys/quilt,kklin/kelda,ejj/quilt-dev,kklin/kelda,ejj/quilt-dev,NetSys/quilt,kayousterhout/quilt,kklin/kelda,kayousterhout/quilt,kklin/kelda,kklin/quilt,quilt/quilt,NetSys/quilt,ejj/quilt-dev,NetSys/quilt,kelda/kelda,kayousterhout/quilt,quilt/quilt