Summary:
This patch allows db_bench to initialize it's RocksDB Options via a
options file, specified by the --options_file flag.  Note that if
--options_file flag is set, then it has higher priority than the
command-line argument.

Test Plan: db_bench_tool_test

Reviewers: sdong, IslamAbdelRahman, kradhakrishnan, yiwu, andrewkr

Reviewed By: andrewkr

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D58533