LU-2253 tests: get proper free indoes

LU-2253 tests: get proper free indoes

In the DNE environment, usually we want to get free indoes on the
master MDT but not the whole filesystem.

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I3bbae177ba556eaa7a7cdb582448c94d24c57bfb
Reviewed-on: http://review.whamcloud.com/7375
Tested-by: Hudson
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>