LU-2840 tests: Clean the environment for test.

Use unique directory for sanityn test_21. Avoid it
impacted by previous test.

Signed-off-by: yang sheng <yang.sheng@intel.com>
Change-Id: I36a06605732bb0048c25c622d4c8f1192d761a88
Reviewed-on: http://review.whamcloud.com/5556
Tested-by: Hudson
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>