Add support for running oltpbenchmark as an external executable cmd.

[xunzhang]

Currently, the command oltpbenchmark can only run in the home folder of the project. If user wants to run oltpbenchmark as an external command(which could be easily integrated into the code), they should fix several hard-coded places.

Here, I add the OLTPBENCH_HOME environmental variable to support this. There is no pollution or previous logic(OLTPBENCH_HOME can be empty in case of not setting it).

Usage: export OLTPBENCH_HOME=~/oltpbench/