Small refactoring to pass SparkEnv into Executor rather than creating SparkEnv in Executor.

This consolidates some code path and makes constructor arguments simpler for a few classes.

Author: Reynold Xin <rxin@databricks.com>

Closes #3738 from rxin/sparkEnvDepRefactor and squashes the following commits:

82e02cc [Reynold Xin] Fixed couple bugs.
217062a [Reynold Xin] Code review feedback.
bd00af7 [Reynold Xin] Small refactoring to pass SparkEnv into Executor rather than creating SparkEnv in Executor.
[SPARK-4889] update history server example cmds

Author: Ryan Williams <ryan.blake.williams@gmail.com>

Closes #3736 from ryan-williams/hist and squashes the following commits:

421d8ff [Ryan Williams] add another random typo fix
76d6a4c [Ryan Williams] remove hdfs example
a2d0f82 [Ryan Williams] code review feedback
9ca7629 [Ryan Williams] [SPARK-4889] update history server example cmds