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.