tests/perf_fast_forward: make data directory location configurable
tests/perf_fast_forward: make data directory location configurable

perf_fast_forward populates perf_fast_forward_output with some data and
then runs performance tests that read it. That makes the disk a
significant factor in the final result and may make the results less
repeatable. This patch adds a flag that allows setting the location
of the data directory so that the user can opt for a less noisy disk
or a ramdisk.

scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla