[Spark-4848] Allow different Worker configurations in standalone cluster

This refixes #3699 with the latest code.
This fixes SPARK-4848

I've changed the stand-alone cluster scripts to allow different workers to have different numbers of instances, with both port and web-ui port following allong appropriately.

I did this by moving the loop over instances from start-slaves and stop-slaves (on the master) to start-slave and stop-slave (on the worker).

Wile I was at it, I changed SPARK_WORKER_PORT to work the same way as SPARK_WORKER_WEBUI_PORT, since the new methods work fine for both.

Author: Nathan Kronenfeld <nkronenfeld@oculusinfo.com>

Closes #5140 from nkronenfeld/feature/spark-4848 and squashes the following commits:

cf5f47e [Nathan Kronenfeld] Merge remote branch 'upstream/master' into feature/spark-4848
044ca6f [Nathan Kronenfeld] Documentation and formatting as requested by by andrewor14
d739640 [Nathan Kronenfeld] Move looping through instances from the master to the workers, so that each worker respects its own number of instances and web-ui port