LU-3053 tests: replay-ost-single/6 wait for delete longer

LU-3053 tests: replay-ost-single/6 wait for delete longer

Add similar code from test 7 to test 6 to wait for delete thread and
add variable wait for kbytesfree to settle down.

Test-Parameters: testlist=replay-ost-single mdtfilesystemtype=zfs  mdsfilesystemtype=zfs ostfilesystemtype=zfs
Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: I89c1270ac5196ca43c17f3a5bd722c0555960065
Reviewed-on: http://review.whamcloud.com/5875
Tested-by: Hudson
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Li Wei <wei.g.li@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>