HOTFIX: Fix concurrency issue in FlumePollingStreamSuite.

This has been failing on master. One possible cause is that the port
gets contended if multiple test runs happen concurrently and they
hit this test at the same time. Since this test takes a long time
(60 seconds) that's very plausible. This patch randomizes the port
used in this test to avoid contention.
MAINTENANCE: Automated closing of pull requests.

This commit exists to close the following pull requests on Github:

Closes #706 (close requested by 'pwendell')
Closes #453 (close requested by 'pwendell')
Closes #557 (close requested by 'tdas')
Closes #495 (close requested by 'tdas')
Closes #1232 (close requested by 'pwendell')
Closes #82 (close requested by 'pwendell')
Closes #600 (close requested by 'pwendell')
Closes #473 (close requested by 'pwendell')
Closes #351 (close requested by 'pwendell')
[HOTFIX] Do not throw NPE if spark.test.home is not set

`spark.test.home` was introduced in #1734. This is fine for SBT but is failing maven tests. Either way it shouldn't throw an NPE.

Author: Andrew Or <andrewor14@gmail.com>

Closes #1739 from andrewor14/fix-spark-test-home and squashes the following commits:

ce2624c [Andrew Or] Do not throw NPE if spark.test.home is not set