SPARK-4338. [YARN] Ditch yarn-alpha.

Sorry if this is a little premature with 1.2 still not out the door, but it will make other work like SPARK-4136 and SPARK-2089 a lot easier.

Author: Sandy Ryza <sandy@cloudera.com>

Closes #3215 from sryza/sandy-spark-4338 and squashes the following commits:

1c5ac08 [Sandy Ryza] Update building Spark docs and remove unnecessary newline
9c1421c [Sandy Ryza] SPARK-4338. Ditch yarn-alpha.