LU-3503 test: only create single directory by test_mkdir()

LU-3503 test: only create single directory by test_mkdir()

Force test_mkdir() to create single directory instead of
multiple directories.

Signed-off-by: Liu Ying <emoly.liu@intel.com>
Change-Id: I222f2bc5e6578b1cd8c4049a10f5ca1790c2c5ad
Reviewed-on: http://review.whamcloud.com/6796
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Robert Read <robert.read@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>