[SPARK-2778] [yarn] Add yarn integration tests.

This patch adds a couple of, currently, very simple integration tests
to make sure both client and cluster modes are working. The tests don't
do much yet other than run a simple job, but the plan is to enhance
them after we get the framework in.

The cluster tests are noisy, so redirect all log output to a file
like other tests do. Copying the conf around sucks but it's less
work than messing with maven/sbt and having to clean up other
projects.

Note the test is only added for yarn-stable. The code compiles
against yarn-alpha but there are two issues I ran into that I
could not overcome:
- an old netty dependency kept creeping into the classpath and
  causing akka to not work, when using sbt; the old netty was
  correctly suppressed under maven.
- MiniYARNCluster kept failing to execute containers because it
  did not create the NM's local dir itself; this is apparently
  a known behavior, but I'm not sure how to work around it.

None of those issues are present with the stable Yarn.

Also, these tests are a little slow to run. Apparently Spark doesn't
yet tag tests (so that these could be isolated in a "slow" batch),
so this is something to keep in mind.
Prior to this test it was not used. SPARK_CLASSPATH is also deprecated.

The new test uses `spark.{driver|executor}.extraClassPath` instead, to make sure they work correctly, and those two settings also conflict with SPARK_CLASSPATH.