Use basho_bench config for riak tests

Use the basho_bench riak test config.  It plays nice with our
internal testers.
Remove unused env var

This env var has been replaced with `yz_dir` and `basho_bench` riak
test config vars.
Remove unnecessary code loading
Merge pull request #204 from basho/refactor/rz/rt-bench

Use `basho_bench` riak test config