[Bugfix] Tachyon file cleanup logical error

Should lookup `shutdownDeleteTachyonPaths` instead of `shutdownDeletePaths`. Together with a minor style clean up: `find {...}.isDefined` to `exists {...}`.

Author: Cheng Lian <lian.cs.zju@gmail.com>

Closes #575 from liancheng/tachyonFix and squashes the following commits:

deb8f31 [Cheng Lian] Fixed logical error in when cleanup Tachyon files and minor style cleanup
SPARK-1689 AppClient should indicate app is dead() when removed

Previously, we indicated disconnected(), which keeps the application in a limbo state where it has no executors but thinks it will get them soon.

This is a bug fix that hopefully can be included in 1.0.

Author: Aaron Davidson <aaron@databricks.com>

Closes #605 from aarondav/appremoved and squashes the following commits:

bea02a2 [Aaron Davidson] SPARK-1689 AppClient should indicate app is dead() when removed