[SPARK-1010] Clean up uses of System.setProperty in unit tests

Several of our tests call System.setProperty (or test code which implicitly sets system properties) and don't always reset/clear the modified properties, which can create ordering dependencies between tests and cause hard-to-diagnose failures.

This patch removes most uses of System.setProperty from our tests, since in most cases we can use SparkConf to set these configurations (there are a few exceptions, including the tests of SparkConf itself).

For the cases where we continue to use System.setProperty, this patch introduces a `ResetSystemProperties` ScalaTest mixin class which snapshots the system properties before individual tests and to automatically restores them on test completion / failure.  See the block comment at the top of the ResetSystemProperties class for more details.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #3739 from JoshRosen/cleanup-system-properties-in-tests and squashes the following commits:

0236d66 [Josh Rosen] Replace setProperty uses in two example programs / tools
3888fe3 [Josh Rosen] Remove setProperty use in LocalJavaStreamingContext
4f4031d [Josh Rosen] Add note on why SparkSubmitSuite needs ResetSystemProperties
4742a5b [Josh Rosen] Clarify ResetSystemProperties trait inheritance ordering.
0eaf0b6 [Josh Rosen] Remove setProperty call in TaskResultGetterSuite.
7a3d224 [Josh Rosen] Fix trait ordering
3fdb554 [Josh Rosen] Remove setProperty call in TaskSchedulerImplSuite
bee20df [Josh Rosen] Remove setProperty calls in SparkContextSchedulerCreationSuite
655587c [Josh Rosen] Remove setProperty calls in JobCancellationSuite
3f2f955 [Josh Rosen] Remove System.setProperty calls in DistributedSuite
cfe9cce [Josh Rosen] Remove use of system properties in SparkContextSuite
8783ab0 [Josh Rosen] Remove TestUtils.setSystemProperty, since it is subsumed by the ResetSystemProperties trait.
633a84a [Josh Rosen] Remove use of system properties in FileServerSuite
25bfce2 [Josh Rosen] Use ResetSystemProperties in UtilsSuite
1d1aa5a [Josh Rosen] Use ResetSystemProperties in SizeEstimatorSuite
dd9492b [Josh Rosen] Use ResetSystemProperties in AkkaUtilsSuite
b0daff2 [Josh Rosen] Use ResetSystemProperties in BlockManagerSuite
e9ded62 [Josh Rosen] Use ResetSystemProperties in TaskSchedulerImplSuite
5b3cb54 [Josh Rosen] Use ResetSystemProperties in SparkListenerSuite
0995c4b [Josh Rosen] Use ResetSystemProperties in SparkContextSchedulerCreationSuite
c83ded8 [Josh Rosen] Use ResetSystemProperties in SparkConfSuite
51aa870 [Josh Rosen] Use withSystemProperty in ShuffleSuite
60a63a1 [Josh Rosen] Use ResetSystemProperties in JobCancellationSuite
14a92e4 [Josh Rosen] Use withSystemProperty in FileServerSuite
628f46c [Josh Rosen] Use ResetSystemProperties in DistributedSuite
9e3e0dd [Josh Rosen] Add ResetSystemProperties test fixture mixin; use it in SparkSubmitSuite.
4dcea38 [Josh Rosen] Move withSystemProperty to TestUtils class.