[SPARK-529][CORE][YARN] Add type-safe config keys to SparkConf.

This is, in a way, the basics to enable SPARK-529 (which was closed as
won't fix but I think is still valuable). In fact, Spark SQL created
something for that, and this change basically factors out that code
and inserts it into SparkConf, with some extra bells and whistles.

To showcase the usage of this pattern, I modified the YARN backend
to use the new config keys (defined in the new `config` package object
under `o.a.s.deploy.yarn`). Most of the changes are mechanic, although
logic had to be slightly modified in a handful of places.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #10205 from vanzin/conf-opts.