[SPARK-23417][PYTHON] Fix the build instructions supplied by exception...

[SPARK-23417][PYTHON] Fix the build instructions supplied by exception messages in python streaming tests

## What changes were proposed in this pull request?

Fix the build instructions supplied by exception messages in python streaming tests.

I also added -DskipTests to the maven instructions to avoid the 170 minutes of scala tests that occurs each time one wants to add a jar to the assembly directory.

## How was this patch tested?

- clone branch
- run build/sbt package
- run python/run-tests --modules "pyspark-streaming" , expect error message
- follow instructions in error message. i.e., run build/sbt assembly/package streaming-kafka-0-8-assembly/assembly
- rerun python tests, expect error message
- follow instructions in error message. i.e run build/sbt -Pflume assembly/package streaming-flume-assembly/assembly
- rerun python tests, see success.
- repeated all of the above for mvn version of the process.

Author: Bruce Robbins <bersprockets@gmail.com>

Closes #20638 from bersprockets/SPARK-23417_propa.