[SPARK-8302] Support heterogeneous cluster install paths on YARN.

Some users have Hadoop installations on different paths across
their cluster. Currently, that makes it hard to set up some
configuration in Spark since that requires hardcoding paths to
jar files or native libraries, which wouldn't work on such a cluster.

This change introduces a couple of YARN-specific configurations
that instruct the backend to replace certain paths when launching
remote processes. That way, if the configuration says the Spark
jar is in "/spark/spark.jar", and also says that "/spark" should be
replaced with "{{SPARK_INSTALL_DIR}}", YARN will start containers
in the NMs with "{{SPARK_INSTALL_DIR}}/spark.jar" as the location
of the jar.

Coupled with YARN's environment whitelist (which allows certain
env variables to be exposed to containers), this allows users to
support such heterogeneous environments, as long as a single
replacement is enough. (Otherwise, this feature would need to be
extended to support multiple path replacements.)

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #6752 from vanzin/SPARK-8302 and squashes the following commits:

4bff8d4 [Marcelo Vanzin] Add docs, rename configs.
0aa2a02 [Marcelo Vanzin] Only do replacement for paths that need it.
2e9cc9d [Marcelo Vanzin] Style.
a5e1f68 [Marcelo Vanzin] [SPARK-8302] Support heterogeneous cluster install paths on YARN.