[SPARK-4032] Deprecate YARN alpha support in Spark 1.2

Author: Prashant Sharma <prashant.s@imaginea.com>

Closes #2878 from ScrapCodes/SPARK-4032/deprecate-yarn-alpha and squashes the following commits:

17e9857 [Prashant Sharma] added deperecated comment to Client and ExecutorRunnable.
3a34b1e [Prashant Sharma] Updated docs...
4608dea [Prashant Sharma] [SPARK-4032] Deprecate YARN alpha support in Spark 1.2