[SPARK-6186] [EC2] Make Tachyon version configurable in EC2 deployment script

This PR comes from Tachyon community to solve the issue:
https://tachyon.atlassian.net/browse/TACHYON-11

An accompanying PR is in mesos/spark-ec2:
https://github.com/mesos/spark-ec2/pull/101

Author: cheng chang <myairia@gmail.com>

Closes #4901 from uronce-cc/master and squashes the following commits:

313aa36 [cheng chang] minor re-wording
fd2a48e [cheng chang] Remove Tachyon when deploying through git hash
1d53c5c [cheng chang] add default value to --tachyon-version
6f8887e [cheng chang] make tachyon version configurable