LU-2955 tests: make replay-ost-single/8b SLOW for ZFS

LU-2955 tests: make replay-ost-single/8b SLOW for ZFS

This test runs dd with osync, which is currently a VERY slow
operation, so mark it slow until LU-2887 is addressed.

Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: I3f55278490bd3c2e3bce8a2de58828a3a716b2fe
Reviewed-on: http://review.whamcloud.com/5767
Tested-by: Hudson
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>