[SPARK-2524] missing document about spark.deploy.retainedDrivers

https://issues.apache.org/jira/browse/SPARK-2524
The configuration on spark.deploy.retainedDrivers is undocumented but actually used
https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/deploy/master/Master.scala#L60

Author: lianhuiwang <lianhuiwang09@gmail.com>
Author: Wang Lianhui <lianhuiwang09@gmail.com>
Author: unknown <Administrator@taguswang-PC1.tencent.com>

Closes #1443 from lianhuiwang/SPARK-2524 and squashes the following commits:

64660fd [Wang Lianhui] address pwendell's comments
5f6bbb7 [Wang Lianhui] missing document about spark.deploy.retainedDrivers
44a3f50 [unknown] Merge remote-tracking branch 'upstream/master'
eacf933 [lianhuiwang] Merge remote-tracking branch 'upstream/master'
8bbfe76 [lianhuiwang] Merge remote-tracking branch 'upstream/master'
480ce94 [lianhuiwang] address aarondav comments
f2b5970 [lianhuiwang] bugfix worker DriverStateChanged state should match DriverState.FAILED
(cherry picked from commit 4da01e3813f0a0413fe691358c14278bbd5508ed)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>