[SPARK-2703][Core]Make Tachyon related unit tests execute without deploying a Tachyon system locally.

Make Tachyon related unit tests execute without deploying a Tachyon system locally.

Author: RongGu <gurongwalker@gmail.com>

Closes #3030 from RongGu/SPARK-2703 and squashes the following commits:

ad08827 [RongGu] Make Tachyon related unit tests execute without deploying a Tachyon system locally

(cherry picked from commit bd86cb1738800a0aa4c88b9afdba2f97ac6cbf25)
Signed-off-by: Patrick Wendell <pwendell@gmail.com>