Rename testExecutorEnvs --> executorEnvs.

This was changed, but in fact, it's used for things other than tests.
So I've changed it back.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #747 from pwendell/executor-env and squashes the following commits:

36a60a5 [Patrick Wendell] Rename testExecutorEnvs --> executorEnvs.
SPARK-1623: Use File objects instead of String's in HTTPBroadcast

This seems strictly better, and I think it's justified only the grounds of
clean-up. It might also fix issues with path conversions, but I haven't
yet isolated any instance of that happening.

/cc @srowen @tdas

Author: Patrick Wendell <pwendell@gmail.com>

Closes #749 from pwendell/broadcast-cleanup and squashes the following commits:

d6d54f2 [Patrick Wendell] SPARK-1623: Use File objects instead of string's in HTTPBroadcast