LU-2297 test: fix replay-single test_74

LU-2297 test: fix replay-single test_74

unmount client before stopping OST, otherwise if client has dirty
data belongig to that OST, client umount will try to wait for
the data flushing back to the unavailable OST and waits forever
for the OST to come back.

Signed-off-by: Oleg Drokin <green@whamcloud.com>
Change-Id: I8af14134b5f417dfd96baef14e02d9f4c8912f6f
Reviewed-on: http://review.whamcloud.com/4717
Tested-by: Hudson
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>