Use .riak_test.config parameter as alternative to YZ_BENCH_DIR
Fix typos and build a proper path as a string
Replace YZ_BENCH_DIR completely with yz_dir RT config parameter
Update yz_dir to reflect full path to YZ in testing location
Merge pull request #203 from basho/fix/bch/rt-env-var

Make YZ_BENCH_DIR environment variable optional