LU-2903 tests: calculation of available space

LU-2903 tests: calculation of available space

Better debug logging for statfs path.
Account for log size in space calculation.
Sleep slightly longer to wait for good statfs because zfs doesn't
count uncommitted changes in available space.

Test-Parameters: mdsfilesystemtype=zfs ostfilesystemtype=zfs   mdtfilesystemtype=zfs testlist=replay-ost-single
Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: Ib4b5a7c645d6b2a630dcc729483422c8b3a095db
Reviewed-on: http://review.whamcloud.com/5662
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>