LU-2620 tests: Flush changes from test 5 in replay-ost-single

LU-2620 tests: Flush changes from test 5 in replay-ost-single

Use wait_delete_completed_mds to flush out and wait for all the
effects on kbytesavail from test 5 to finish before test 6 starts.

Test-Parameters: testlist=replay-ost-single envdefinitions=SLOW=yes

Signed-off-by: Bob Glossman <bob.glossman@intel.com>
Change-Id: Id37cf23402dd609e6743c36a1aff8e71dd246776
Reviewed-on: http://review.whamcloud.com/5042
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Minh Diep <minh.diep@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>