LU-8641 tests: tests: speedup metabench

LU-8641 tests: tests: speedup metabench

With huge number of nfiles created by metabench
rm -rf called at the end of run_metabench() takes a very long
time.
Sine metabench cleanup is also very slow, metabench
execution without -k option is useless to use also.
Patch adds mbench_CLEANUP mode, default value is true.
When set to false -- metabench testdir is just renamed to avoid
files/dirs deletion.

Seagate-bug-id: MRP-3454
Signed-off-by: Elena Gryaznova <elena.gryaznova@seagate.com>
Reviewed-by: Alexander Zarochentsev <alexander.zarochentsev@seagate.com>
Reviewed-by: Vitaly Fertman <vitaly.fertman@seagate.com>
Change-Id: Iaa130785d27dc3a87f642723fcaad6dc5e748440
Reviewed-on: https://review.whamcloud.com/22852
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>