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.

Signed-off-by: Niu Yawei <niu@whamcloud.com>
Change-Id: I524d42a5e792a1b29db52166e92624bf42312c3f
Reviewed-on: http://review.whamcloud.com/4705
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>