Summary:
This patch does the following improvement on the regression_test.sh
* allows db_bench being executed at a remost host while storing the
  benchmark results locally.
* kills all db_bench related processes before running db_bench
* better error handling.

Test Plan:
1. Run regression_test.sh both locally and remotely
2. Run multiple regression_test.sh at the same time and make sure
   i. Only one runs successfully.
   ii. The one that runs successfully will kill all other db_bench
       processes before it runs any benchmark.

Reviewers: sdong, yiwu, gunnarku

Reviewed By: gunnarku

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D58611