SPARK-1680: use configs for specifying environment variables on YARN

[tgravescs]

Note that this also documents spark.executorEnv.* which to me means its public. If we don't want that please speak up.

[SparkQA]

QA tests have started for PR 1512. This patch merges cleanly.
View progress: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder/16912/consoleFull

[SparkQA]

QA results for PR 1512:
- This patch PASSES unit tests.
- This patch merges cleanly
- This patch adds no public classes

For more information see test ouptut:
https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder/16912/consoleFull

[tgravescs]

I need to update so the documentation shows up properly

[SparkQA]

QA tests have started for PR 1512. This patch merges cleanly.
View progress: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder/16916/consoleFull

[SparkQA]

QA tests have started for PR 1512. This patch merges cleanly.
View progress: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder/16917/consoleFull

[SparkQA]

QA results for PR 1512:
- This patch PASSES unit tests.
- This patch merges cleanly
- This patch adds no public classes

For more information see test ouptut:
https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder/16917/consoleFull

[tgravescs]

@pwendell are you ok with exposing the spark.executorEnv. config?

[mateiz]

@tgravescs I think it's fine to expose that. We actually already document it in the comments of SparkConf.setExecutorEnv and such.