Summary:
This patch does the following improvement for regression_test.sh
* Allow regression_test.sh to specify OPTIONS_FILE.
* Add header comments that includes examples on how to run the script
  and introduce all configurable parameters.
* bug fix.

Test Plan: Run the example commands in the header comments of regression_test.sh

Reviewers: sdong, yiwu, gunnarku

Reviewed By: gunnarku

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D59175