LU-3973 tests: use -P option of df to avoid line breaks

LU-3973 tests: use -P option of df to avoid line breaks

Sometimes df command output requires to break lines to
allow for columns alignement. This causes fields parsing
to fail. Use -P option to select compatibility output
and ensure one single line format per file-system is used.

Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Change-Id: Ibc9d4b8f50857e680a5590dbbfe312fce137bcb6
Reviewed-on: http://review.whamcloud.com/7915
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: jacques-Charles Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Reviewed-by: James Nunez <james.a.nunez@intel.com>