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

[bersprockets]

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.

[HyukjinKwon]

ok to test

[HyukjinKwon]

add to whitelist

[SparkQA]

Test build #87545 has finished for PR 20638 at commit 8241313.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[bersprockets]

Do we want to merge this or does it need more eyes?

[HyukjinKwon]

Merged to master.

[bersprockets]

@HyukjinKwon Oh, thanks. I guess I learned what "approved these changes" means.