[SPARK-1022][Streaming] Add Kafka real unit test

This PR is a updated version of (https://github.com/apache/spark/pull/557) to actually test sending and receiving data through Kafka, and fix previous flaky issues.

@tdas, would you mind reviewing this PR? Thanks a lot.

Author: jerryshao <saisai.shao@intel.com>

Closes #1751 from jerryshao/kafka-unit-test and squashes the following commits:

b6a505f [jerryshao] code refactor according to comments
5222330 [jerryshao] Change JavaKafkaStreamSuite to better test it
5525f10 [jerryshao] Fix flaky issue of Kafka real unit test
4559310 [jerryshao] Minor changes for Kafka unit test
860f649 [jerryshao] Minor style changes, and tests ignored due to flakiness
796d4ca [jerryshao] Add real Kafka streaming test
SPARK-1528 - spark on yarn, add support for accessing remote HDFS

Add a config (spark.yarn.access.namenodes) to allow applications running on yarn to access other secure HDFS cluster.  User just specifies the namenodes of the other clusters and we get Tokens for those and ship them with the spark application.

Author: Thomas Graves <tgraves@apache.org>

Closes #1159 from tgravescs/spark-1528 and squashes the following commits:

ddbcd16 [Thomas Graves] review comments
0ac8501 [Thomas Graves] SPARK-1528 - add support for accessing remote HDFS
SPARK-1890 and SPARK-1891- add admin and modify acls

It was easier to combine these 2 jira since they touch many of the same places.  This pr adds the following:

- adds modify acls
- adds admin acls (list of admins/users that get added to both view and modify acls)
- modify Kill button on UI to take modify acls into account
- changes config name of spark.ui.acls.enable to spark.acls.enable since I choose poorly in original name. We keep backwards compatibility so people can still use spark.ui.acls.enable. The acls should apply to any web ui as well as any CLI interfaces.
- send view and modify acls information on to YARN so that YARN interfaces can use (yarn cli for killing applications for example).

Author: Thomas Graves <tgraves@apache.org>

Closes #1196 from tgravescs/SPARK-1890 and squashes the following commits:

8292eb1 [Thomas Graves] review comments
b92ec89 [Thomas Graves] remove unneeded variable from applistener
4c765f4 [Thomas Graves] Add in admin acls
72eb0ac [Thomas Graves] Add modify acls
[SPARK-2860][SQL] Fix coercion of CASE WHEN.

Author: Michael Armbrust <michael@databricks.com>

Closes #1785 from marmbrus/caseNull and squashes the following commits:

126006d [Michael Armbrust] better error message
2fe357f [Michael Armbrust] Fix coercion of CASE WHEN.
[SPARK-2859] Update url of Kryo project in related docs

JIRA Issue: https://issues.apache.org/jira/browse/SPARK-2859

Kryo project has been migrated from googlecode to github, hence we need to update its URL in related docs such as tuning.md.

Author: Guancheng (G.C.) Chen <chenguancheng@gmail.com>

Closes #1782 from gchen/kryo-docs and squashes the following commits:

b62543c [Guancheng (G.C.) Chen] update url of Kryo project
use configs for specifying environment variables on YARN
try fix docs
fix docs
fix documentation
more doc changes