LU-797 tests: process lfs df output properly

LU-797 tests: process lfs df output properly

There is a space in the "filesystem summary" of lfs df output,
which could cause trouble for the parsing script, test_23b of
ost-pool.sh is an example.

We introduced lfs_df() function to remove this space internally,
this will allow fixing the "lfs df" output line in the future.

port of master patch fb1a1ec6300a5ec3925b725d5d2b783314dff3f8

Signed-off-by: Niu Yawei <niu@whamcloud.com>
Signed-off-by: Liu Ying <emoly.liu@intel.com>
Change-Id: Iba57b4aac714e28ebdd985f15ff3e2444e64e007
Reviewed-on: http://review.whamcloud.com/4784
Tested-by: Hudson
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>